MailMate Homepage: https://freron.com/?ref=receipts
Mails or attachments can directly be dragged on Receipts for actual import.
For mails in general it imports the first attached PDF file, otherwise it transforms the email body automatically into a PDF which is very useful with e.g. iTunes and Google Play invoices.
In MailMate's settings you can choose to activate the "Receipts" bundle. You may use it from the main menu via
Add.. or the related keyboard shortcut.
You may also set up a mail rule for certain types of repeating mails you want to import to Receipts automatically. You can either choose to send those mails to Receipts via the before mentioned bundle command or by exporting it to an observed folder.
See also: Integration: Apple Mail