Python Example: Creating an Invoice

import base64
import hashlib
import json
import os
import time

import requests


def canonical_json(payload):
    encoded = json.dumps(
        payload,
        ensure_ascii=False,
        separators=(',', ':'),
        allow_nan=False,
    )
    escaped = []
    for character in encoded:
        codepoint = ord(character)
        if character == '/':
            escaped.append(r'\/')
        elif codepoint < 0x80:
            escaped.append(character)
        elif codepoint <= 0xFFFF:
            escaped.append(f'\\u{codepoint:04x}')
        else:
            codepoint -= 0x10000
            escaped.append(f'\\u{0xD800 + (codepoint >> 10):04x}')
            escaped.append(f'\\u{0xDC00 + (codepoint & 0x3FF):04x}')
    return ''.join(escaped)


def legacy_sign(body, api_key):
    payload_base64 = base64.b64encode(body.encode('utf-8')).decode('ascii')
    return hashlib.md5((payload_base64 + api_key).encode('utf-8')).hexdigest()


merchant_id = os.environ['COINSSEND_MERCHANT_ID']
api_key = os.environ['COINSSEND_API_KEY']
payload = {
    'order_id': f'order_{int(time.time())}',
    'amount': '100.50',
    'is_customer_fee': False,
    'is_customer_network_fee': False,
    'allow_card_payments': True,
    'success_url': 'https://merchant.example/payment-success',
    'cancel_url': 'https://merchant.example/payment-cancelled',
}
body = canonical_json(payload)

response = requests.post(
    'https://api.coinssend.com/v1/invoices',
    headers={
        'Content-Type': 'application/json',
        'Merchant': merchant_id,
        'Sign': legacy_sign(body, api_key),
    },
    data=body,
    timeout=15,
)
result = response.json()

if not response.ok:
    raise RuntimeError(result.get('error') or result.get('message') or f'HTTP {response.status_code}')

print(result['data']['code'], result['data']['url'])

Use data=body, not json=payload: the canonical string is both the signed payload and the sent body. Persist the returned identifiers. A duplicate order_id returns 409 without replaying the original response.