expenses without invoice

If you are not subject to VAT, I believe it is sufficient to upload a pDF from that email when you record the expense. Or the receipt that Instagram creates: https://www.facebook.com/help/instagram/789724481572641</

This way, you have proof that you incurred that expense.

If you are subject to VAT, you will generally need an invoice from Instagram that clearly states the word 'Invoice' and also includes your company details + your VAT number (which is not always easy to obtain).