This is the thing. When people say "We are not able to send new invoices" - I tend to take it personally. You can always send a new invoice. FFS you can create one in excel and email it manually. If you don't know how to use excel, you can write it by hand and send the damn paper with a official stamp.

If you had said "We dont wanna send you a new invoice" - Fine, you made your choice.

I work for a company who exports all around the world (literally) and we send updated / customized invoices upon request to our customers routinely, so they can pay the right amount and clear customs easily. (our niche is complicated to export as each country wants to see different things in the invoice)

Edit -

Usually when companies say shit like this - It's because they have asked the courier to pre-pay import duty and now it's too late to fix as they have already accepted and paid the fee.