"Choose the right price for yourself for a long weekend." - the supplier
and
"Besides free freight the offer was that we could choose our own price!" - you
Maybe you need to do some more English classes ! OK, I'm joking, but an Englishman would never take that as you choosing your own price. Basically, all they are telling you is that you can submit a list of parts you want, and they will quote a price for them in total, with maybe/probably a discount, rather than you picking each one off their website and totalling it up to get a price. It doesn't sound unreasonable to me.