Open Invoices in the sidebar (needs catalog:manage).
Create and send
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Choose to create an invoice.
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Enter the client: company name, email, tax id and address as needed.
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Add line items with a description and amount in your currency.
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Add payment terms or a message, then issue it.

Mayarin emails the client a pay link when you gave an email. They pay with whatever asset they hold; you settle in your stablecoin.
Good to know
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Your city and country print on every invoice. Set them in Settings → Business profile first.
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An issued invoice has a due date. Unpaid past it, it is shown as overdue.
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A paid invoice produces a receipt for the client.
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Recurring billing? Use subscriptions, which issue one invoice per cycle for you.