If you dont need deliv conf then its got to do with the thickness of the envelope...this is the biggest variable part when using a clerk vs online

Some clerks will measure thickness and some don't

Regular envelopes without deliv conf can only be 1/4" thick to get standard rates, then you need to jump up to small package rates

Regular envelopes with deliv conf also need to be charged as a small pkg because to use deliv conf it must be at least 1/2 thick