Double-charged customers, duplicate webhooks
Payment and crypto callbacks arrive more than once, out of order, or after a timeout retry. Without idempotency keys and a single source of truth, a retried webhook quietly credits a balance twice.
FINTECH DEVELOPMENT
Fintech Development Company — Payments, Crypto & Wallets
Ostohlo builds fintech systems — payment gateways, crypto payment integration, wallets and transaction dashboards — engineered for correctness under money-moving load.
9products in our ecosystem
5verticals
3languages
100%custom code
You are moving money, so the bar is different. A fintech product must be right when a webhook arrives twice, when a network partitions mid-transfer, and when a customer disputes a balance months later. Ostohlo is an international product web studio — we build systems, not pages — and we treat payments, crypto rails and wallets as ledgers first, interfaces second. See the wider context in our services.
Launching a new payment gateway, adding crypto payment integration to an existing SaaS, or replacing spreadsheet reconciliation with a real dashboard — we scope the architecture, build it, and connect the providers you depend on. We are engineers, not a licensing firm: we build the technology and connect the rails, working alongside your compliance and legal advisers rather than replacing them.
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What usually goes wrong
Crypto integrations that leak keys
Keeping hot private keys on the web server 'to make refunds easy' turns one server compromise into an emptied wallet. Address derivation, signing and custody need a design that assumes the web tier is hostile.
Reconciliation by spreadsheet
When your ledger, your provider's dashboard and your bank statement disagree, someone exports three CSVs at month end and reconciles by hand. It does not scale and it hides losses.
Dashboards that lie under load
A balance that is 'eventually' correct is wrong when a customer is looking at it. Caching, races and read replicas produce transaction views that disagree with the ledger.
Tell us about your task — we'll come back with architecture and an estimate within 24 hours.
Get an estimate(02)
What we build
Payment gateway development
Payment gateways built end to end — invoice creation, provider integration, callbacks, retries, refunds and payouts — with an append-only ledger as the authoritative record rather than a mutable balance column.
Crypto & USDT-TON integration
Crypto payment integration including USDT-on-TON: unique per-invoice deposit addresses, on-chain confirmation tracking, and a settlement model that never trusts a single unconfirmed event.
Wallets & balances
Multi-currency wallets on double-entry accounting, so every credit has a matching debit and the total is always provable. Holds, reversals and fees are modelled explicitly, not patched in later.
Transaction dashboards
Operator and customer dashboards that read from the ledger as the source of truth — searchable history, statuses, exports and audit trails — so the number a user sees matches the accounts.
Security & correctness
Signed webhooks as the source of truth, idempotency on every money-moving endpoint, and read-only key handling on public servers so a web-tier breach cannot move funds.
Reconciliation & reporting
Automated reconciliation between your ledger, your PSPs and your on-chain records, with reporting that surfaces mismatches early. Month-end becomes a review, not an archaeology dig.
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Products already in production
Our clearest fintech proof is our own live product. Payora (payora.money) is a crypto payment gateway we built and run: it issues a unique deposit address per invoice, keeps only read-only key material on the server, and treats signed webhooks as the single source of truth for settlement — the exact patterns we bring to client builds. See the wider range of systems we ship in our portfolio.
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How we work
01
Ledger first, UI second
We model the money — accounts, entries, invariants — before we design a single screen. The interface can be rebuilt cheaply; a wrong ledger cannot.
02
Assume every event repeats
Every webhook and callback is designed to be processed twice safely. Idempotency, deduplication and signature verification are baked in from the first commit, not retrofitted after an incident.
03
Least privilege by default
Public servers hold read-only credentials; signing and custody live behind tighter boundaries, so any single compromise stays contained.
04
Integrate, don't reinvent
We connect proven providers and chains rather than rebuilding settlement from scratch, and document every integration so your team can operate and extend it without us.
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Why Ostohlo
- We run our own fintech product, Payora — the patterns we recommend are ones we operate in production, not slideware.
- We build systems, not pages: your ledger and integrations are the deliverable, and the UI serves them.
- Correctness under retries, races and partial failure is designed in from the first commit.
- International, multilingual delivery across the US, UK, Germany, Poland, the Netherlands and the Baltics.
- We integrate providers and chains and hand you documented, operable systems — no lock-in to us.
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FAQ
Fintech projects with us typically start around $20k and scale with scope — a focused gateway or crypto integration at the lower end, a full wallet-and-dashboard platform with reconciliation larger. We scope in phases, so you fund a defined milestone rather than an open-ended promise.
Yes. Crypto payment integration into an existing SaaS or marketplace is one of our most common briefs. We add unique per-invoice addresses, confirmation tracking and signed-webhook settlement without rewriting your core. Our guide on accepting crypto payments on a website walks through the model.
We build the technology and integrate the providers; we are not a legal or licensing firm. We work alongside your compliance and legal advisers, implement the controls they require — KYC/AML hooks, audit trails, data handling — and never present engineering as legal advice.
Because networks retry. A webhook can arrive twice or arrive spoofed. Verifying the signature proves it came from the provider; an idempotency key proves you apply it once. Together they stop double-credits and fraudulent callbacks.
Public-facing servers hold read-only material only — enough to derive addresses and watch the chain, never enough to move funds. Signing and custody live behind tighter boundaries. It is the same design we run in Payora.
A documented, operable system: source code, an authoritative ledger, integrated providers, dashboards and reconciliation, plus runbooks your team can use.
FINTECH DEVELOPMENT
Building a fintech product? Let's scope it properly.
Tell us what you're moving — fiat, crypto, or both — and we'll map the ledger, integrations and controls first. Get in touch and we'll come back with a phased plan and an honest budget.
