Oy vey.
If you go into the post office the mindset the OP seems to have, no wonder there are problems.
I would bet that 90%++ of USPS daily transactions do not involve stamps, so their workflow is optimized for printed labels vs. the old school affixing of stamps. The latter is no longer the norm, and one shouldn't get offended when a postal clerk's first instinct is to affix a printed postage label.
I've found that in general, if you make things easier for them, they will make things easier for you, whereas if you constantly have a chip on your shoulder, they won't want to accommodate you, nor do I blame them.
As previously mentioned, get a cheap digital scale, use the USPS online price calculator, and affix your postage before going to the post office. It saves yourself time, the clerk's time, and the time of all the people waiting behind you in line.
My workflow for my
ebay shipments:
1. Package up and affix my return and address labels.
2. Weigh package, and mark weight on my copy of the invoice.
3. Go to USPS web site, put in origin and destination zip codes, select type of shipment (parcel, letter, etc.) and weight. Add any extra services (insurance, signature confirmation, certified, etc.). Calculate price.
4. Write price on upper-left corner of parcel in ink (the clerks really appreciate this when you are affixing a zillion stamps, as it is a point of reference whether you got the rate correct or whether you made an error or your postal scale is off, rather than standing there counting up stamps).
5. If the shipment is a parcel that gets tracking, apply a tracking number. Yes, you apply the tracking number yourself rather than waiting for the clerk to do it at the post office, by getting pads of tracking numbers from USPS:
https://store.usps.com/store/browse...d=P_LABEL4006. Affix postage.
Now when you get to the post office, you have taken all of the decisions out of the hands of the postal clerks. You have indicated the amount of postage paid, affixed postage, and supplied them a tracking number. The only thing they have to do is scan in the tracking number, confirm the rate and postage amount, cancel stamps, and done.
My packages take about 30 seconds (if that) at the window for them to process and do the acceptance scan. I do most of the work for them, and they know through a long history that I do a lot of domestic and international shipping and know the rate differences for letter vs. letter with nonmachineable surcharge vs. flat vs. parcel. They don't even question my calculations any more.
I make things go faster for them, so they have zero problems accommodating my occasional philatelic requests, e.g., "please hand cancel this one especially neatly".
They have a job to do... don't make things more difficult for them.