Inflatable Boatworks, Seattle Inflatable must mean the Price - ripped off on a $5.50 part Seattle, Washington

Yesterday I purchased a Harkey Roberts pump adapter, common for inflating boats, pontoon boats etcetera. I was charged $20.99 for this adapter. I thought the price was high so I went online to see if I could get it from the manufacture of my boat, Outcast. The price in their catelog was $5.50. I called the store and they told me that I was overcharged, the price should have been $15.00, only $9.50 more than actual retail for the product. When I asked about this, that they surely purchased them for less than retail, $5.50, I was told that the rest was for customer service and if I wanted to drive the 20 miles back to the store they would credit me the $6.00 they felt that they actually overcharged me. Again, "inflatable", must surely mean prices. I am more than willing to pay maybe a $5 premium for the convenience but...................a bit much if you ask me.