Many have been a victim of double spending before and many merchants have already sent their items without knowing that what they received is a double spent bitcoin rendering it valueless.
Most merchants make use of a payment processor that takes care of everything related to the Bitcoin payment option. If the payment processor in question doesn't fully trust an incoming payment due to a low included fee, then they won't "credit" the merchant until that transaction gets its first confirmation. In that regard, as long as the merchant isn't being credited, the merchant won't send out what you bought. If you look at how soft local merchants were back in 2013/2014, where you could buy yourself something to drink and a hamburger, where after that you could just walk out and perform a double spend. It's not like that anymore. They will force you to wait till that first confirmation comes through now, which renders Bitcoin as local currency worthless in that regard. No one is going to wait possibly 30-50 minutes in a store to buy something.