So I bought a downstem at the local headshop here, and I asked for 18.8mm quite specifically, and I thought the downstem looked a little small for 18.8 and I was a dumbass for not asking him to measure it, and go figure when I got home it doesn't fit it's completely unused and I paid for it in cash, should I go in and see if I can get it exchanged? as far as I know no refunds/exchanges on smoking products..
If its completely unused then yes they should remember you, and they should be able to tell its still new.
just talk to them man, if its unused im pretty sure theyd be willing to take it back. if they wont accept an unused downstem, then just start going to a new headshop.
One time I accidentally got a low profile diffuser instead of a high profile diffuser at a head shop that said no exchanges allowed, but I took it back later that day and exchanged it without any issues. They'll exchange it if you haven't used it, because they likely value your business.