it seems that you haven't paid only that your tax return has been received. you can establish an id.me account and make an online payment.
https://www.irs.gov/payments
print out the confirmation in a day or so you probably can go online to your bank account to print out that the IRS has processed payment. that should be satisfactory. the IRS will at some point send you a bill for any penalties and interest since payment was due 4/15. if the other party wants proof that these too were paid, I know of no way to expedite the IRS processing your return and sending you a bill
