Allocating Invoice Payment along with Sumup Fee I can no longer allocate a payment to the invoice and the rest to Sumup account for their fee. Anyone have a solution?

Welcome, and thank you for adding your first post, Colin. 

There are two ways to allocate invoice payment with fees in QuickBooks Online (QBO). I'm here to share them with you.

The first option is to enter the fee when depositing the payment amount.

1. Receive the invoice payment.
   1. Go to the +New icon, then click Receive Payment.
   2. Choose the customer name.
   3. Select Undeposited Funds from the Deposit to dropdown.
   4. Choose the invoice, then enter the payment amount under Amount received.
   5. Click on Save and Close.
2. Deposit the payment to your bank minus the transaction fee.
   1. Click on Bank deposit from the +New icon.
   2. Choose your bank account from the Account dropdown.
   3. Select the payment.
   4. Go to the Add funds to this deposit section, then enter the fee in a negative amount.
   5. Save the transaction.

The second method is to add the service fee to your invoice manually. I'm adding this article as your reference: Manually add service fees to invoices in QuickBooks Online.

Aside from invoices and payments, you can also get more tips while working with your other customer entries from this link: Topics about your company's income and customers.

If you have follow-up questions while tracking your invoice payments with fees, let me know by adding a comment below. I'm always here to help. Have a good one!