Allow for separate return address and "sent from post office" zip codes
A USPS label (like the ones generated by Stamps.Com) has two different sender zip codes - the sender's return address and the post office where the package will be sent from. These are often, but *not* always, the same. I mail from a different zip code because that post office is closer than the one that services my house. Postmasters prefer to have the correct "sent from" Zip Code because the that's how the USPS tracks their performance.
The USPS will send the package regardless. We frequently use different post offices or home pickup depending on which of us is sending the item. My partner and I live in different zip codes for example and he works in another.
The USPS may prefer it but as long as it's got the proper postage they will take it. They actually are required to despite what some will tell you.
Y'all actually already have the second zip code in the seller data. It's Settings->SellerSettings->Shipping&Returns->ItemsShipFrom->ShipsFromPostCode
All you need to do is add that second zip code to the message send to Stamps.Com to generate the postage labels.