Why is ７元/并 written instead of ７元/瓶?
I'd say both you 1 and 2 apply here.

It is is an "abbreviation". A lot of characters can take a more simple form in handwriting, to make writing easier. It is not a correct form. It is not standardized.

Some of the character forms come from an abandoned simplification scheme in China: https://en.wikipedia.org/wiki/Second_round_of_simplified_Chinese_characters, some of them are just made up in daily life.

I'd say I would treat this as incorrect. I do not have much difficulty recognizing the label, with the very specific context where it is used. But without the context, I'm not sure if I can recognize it as a simplification of "瓶".