In my experience, about 95% of payment gateways has some limitations on an «internal ID assigned by the merchant to the transaction».
E.g., these limitations could be enforced by a payment gateway’s database scheme.
The AlphaCommerceHub API reference does not mention any limitations.
Does it mean that MerchantTxnID can have infinite length and can contain any Unicode characters?
http://alpha.pwstaging.com.au/docs/api-integration-guide#request-message-
