For UPS & Fedex, you should always get a receipt. Or any other non government carrier such as DHL
However, USPS doesn't offer shipping receipts to my knowledge for prepaid shipping labels. Only if you purchase the postage in person You can only prove you shipped once the package is scanned and tracking is updated. My local USPS is busy most of the day and even if you stand in line, they aren't going to necessarily scan the package in front of you (instead they'll take it from you and put it into a sorting bag or somewhere in the back). They usually request you to put prepaid labels at the counter, which they'll scan later (they don't always scan them all unfortunately). When GOAT was previously USPS only, one time my package wasn't scanned for the 1st time until it reached Culver City. GOAT was all over my *** and I told them USPS got lazy and didn't bother scanning it. I'd estimate that USPS didn't scan my package at acceptance anywhere from 25-30% of the time, which was way too frequent considering how anal GOAT is about tracking.