So I just got this and I don't see any option where I can spend 500 caps to raise the stores buying capacity, I tried talking to the store owner but to no avail.

Maybe use the build mode, thats how supply lines work.

I think at the bottom it hints what button to press when you are actually looking at what the vendor sells? I can't be certain though, sorry.

Yep. You have to go to the shop interface and press a certain button. Should be a key at the bottom.

Okay thanks for the help.