Handling Fee

I bought a Garmin FR70 from Navworld Friday 6 Nov 2015 and realised that this watch is not compatable with my tablet. I went back to Garmin on Tues 10 Nov 2015 to change this to a watch with a bluetooth and was told I need to pay 15% handling fee which is not accepatable. I have checked it is not on my cashslip which I received when I bought the watch