The QIWI Wallet API specification does not forbid any characters in the bill_id parameter of the PUT api.qiwi.com/api/v2/prv/{prv_id}/bills/{bill_id} request.
The proofs:
- The GitHub-based documentation:
bill_id- unique invoice identifier generated by the merchant (any non-empty string of up to 200 characters)
bill_id- уникальный идентификатор счета в системе провайдера.
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The PDF-based documentation: (REST API specification (v.2.12), page 7):
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in English:

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in Russian:

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