Custom charges? experience, advice?

SL2

Hi I've recently purchased new push rims for my wheelchair from the U.S. and had customs send me a invoice for customs charges and handling fees. As it's designed solely for the disabld and was declared by the sender as an item for the disabled I'm confused to why thy've placed the duty on the item. They won't release the item until the charges are paid and given me the option of paying and filing a claim, disputing the claim which takes a minimum of 6 weeks or have it sent back. As it's an essential item I don't want to have it sent back or disputing it now. The supplier has been sending similar items for years with no problem.
So I'm thinking to pay and then dispute the charges but wanted to know if anyone had any similar experience or advise? is it worth the trouble or should I just cough up and consider the money gone.