Now did you use Amazon because these firms will not sell direct ?
I'm not sure if I understand your question
As a random data point (and probably not the best example), Amazon has the P&S CRB5362-W 20A receptacle for $2.98. Platt has it listed for $9.57. Gordon has it listed for $2.69 with $10.00 shipping.
At a certain point, inertia takes over. I buy a lot from Amazon. The account is already set up. If I can avoid driving somewhere for a few cents more, I will do it.
Also, a while back I knew I was going to be short on 4" boxes. I didn't want to take time to drive to the SH, so I ordered 30 Steel City boxes from Amazon. I have Amazon Prime, so I knew they would be here two days later with free shipping.
You can also get some great deals on tools on Amazon. Using CamelCamelCamel helps.