immigration fees paid (for physical gc) but no confirmation mail...

[dealsone]

on 08jun18, in the iscis immigration fees payment link, i filled in the A66 **** ** numbers & case id BMB **** * & paid $660 immigration fees (for physical gc) on uscis site ($220 X 3 - visa debit card) . after i input all my card details & my ***, there was no acknowledgment/receipt but after 6-7 seconds it displayed a note ?server error 500?
i then checked my card account & it was debited by $660
so, on the uscis payment link, i again input the A66 **** *** number & case id, it then displayed a note:
?The USCIS Immigrant Fee has already been paid for this person. If this person has not received their Green Card yet, create a USCIS Online Account to verify or update their mailing address.?

i still do not have a receipt/confirmation mail from uscis for the payment made.
what proof do i have - if asked to be presented.

[dealsone]

on 08jun18, in the iscis immigration fees payment link, i filled in the A66 **** ** numbers & case id BMB **** * & paid $660 immigration fees (for physical gc) on uscis site ($220 X 3 - visa debit card) . after i input all my card details & my ***, there was no acknowledgment/receipt but after 6-7 seconds it displayed a note ?server error 500?
i then checked my card account & it was debited by $660
so, on the uscis payment link, i again input the A66 **** *** number & case id, it then displayed a note:
?The USCIS Immigrant Fee has already been paid for this person. If this person has not received their Green Card yet, create a USCIS Online Account to verify or update their mailing address.?

i still do not have a receipt/confirmation mail from uscis for the payment made.
what proof do i have - if asked to be presented.

received am email from uscis:
*
'Our system indicates that we received your USCIS Immigrant Fee on June 9, 2018. At this time, USCIS?s systems do not show a payment failure. However, the US Treasury Department payment system used by the USCIS online filing system, Pay.gov, is a third party processor. They may not have notified USCIS of any failure yet, but they could still do so in the near future. If that happens, USCIS will mail you an insufficient funds notice to the US address on file, which will provide instructions on how to reconcile any insufficiency'
*
does it mean, waiting for the 'other shoe to fall...'