Technically, the following part counts as a hidden fee ^{[you should consider mentioning how much it's going to be added]}:

• Quote from: https://payid19.com/dev/invoices/create_invoice
  the price of product is under 0.4 USD will be added fee to price automaticallly

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While I was reading that page, the following part caught my attention... How could your gateway offer such a thing to customers, when there's an expiration timer in the invoice window ^{[based on the screenshots]}?

• Quote from: https://wordpress.org/plugins/payid19-com-payment-gateway/#description
  Payid19.com module allows you to sell with installments on your website with supported cryptocurrencies.