This Ask for Additional Donation prompt can be configured in two places:

1. Fundraising > eCommerce > Edit the desired eCommerce store > 12. Configure Standard Pages > "Cart, Checkout, Single Address Entry Pages": Edit > Ask for Additional Donation: checkbox

AND

2. Edit the eCommerce store > Edit the desired eCommerce store > 12. Configure Standard Pages > "Store Home Page": Edit > Ask for Additional Donation: checkbox

To prevent that message from displaying, both of these checkboxes must be disabled.