Where can I find my receipt/invoice?

You can find all your receipts under Menu => Settings => My Invoices.
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Here's a direct link: https://lnk.bio/manage/invoices

If you have many single accounts, the invoices are available in each individual account.

If you have an Agency account, your invoices are under Menu => Settings => Agency Invoices.

Here's a direct link: https://lnk.bio/manage/agency/invoices