You aren't buying a custom-made bicycle. Generally, these sorts of bikes come in a range of sizes that, simplisticly (and often practically) are measured in XS, S, M, L and XL. They correspond to measurement of the seat tube, as you likely know. In ordering the bike, the staff probably had a pretty good clue as to what sized frame to order with what you regard as cursory measurements.

The task for you and the bike shop comes after delivery of the bike, and that is to work together and negotiate on the fine-tuning of the bike to fit you*. Things like longer or shorter stem, height of the steerer tube, position of the seat fore and aft and height-wise, even changing the seat, and so on will make the bike comfortable, if the basic frame is right.

It sounds as though the bike shop is trying to work with you. Perhaps you should meet them next with the same sort of approach rather than gee-ing yourself up with "assertiveness training".

* There are some caveats. For all we know, you may have some extraordinary body dimensions, such as very short torso and very long inseam, or short arms in comparison to your height... and that can make fit on a production bike somewhat more difficult (frustrating even). I also am assuming that you are female (yes, there are some here who post with female names but are men) and that the bike is the Femme version which means the geometry is more suited to a "standard" woman's measurements.