How do I send an invoice to a customer?

Open an issued invoice. Download PDF gives you the invoice as a PDF: your logo, your company's legal name, address, GSTIN and state, the customer's details (and where it ships to, if different), the place of supply, whether reverse charge applies, each line with its HSN or SAC, a tax summary, the total in figures and words, your bank details and "Authorised signatory". Margin keeps your details as they were on the day you issued it, so a later change of address doesn't change invoices already sent.

Email invoice sends it to the customer with the PDF attached, from your own email domain if you have that add-on. The customer's email, the subject and a short message are filled in for you to change, and you can copy others in. Replies come to your company's email. Every email sent is kept on the invoice, and you can send it again. Margin never emails an invoice by itself.

For the PDF to show how to pay you, set your UPI ID and the bank account to print in Finance settings → Invoices (changing either asks for a code we email you). With a UPI ID, each invoice carries a QR code any UPI app can scan, for the amount still owed, with the invoice number as the payment note, so bank matching can find the invoice when the money arrives.