Don't know about things in the UK, but in the states, each CC has variable rates depending upon the type and volume of the business, how they physically interact with customers, and their past record.
If this was me, I'd be thinking that he's already paying 3-5% for credit card transactions. Comissioning you at 3% for sales gets him an additional sales outlet for no - or even less - addtional cost.
Also, remember that your arrangement with the CC processor is subject to change. Your arrangement with the shop owner needs to reflect that.
One option you have is to take an online order subject to verification. The shop owner then "swipes" the card the next day from his retail location. I imagine that orders would be shipped by the shop owner so combining the verification and distribution stages wouldn't be a terrible hardship.
Just make sure that the process leaves you with the customer contact responsibilities. Keeps the shop owner from claiming the card didn't pass when it actually did.
BTW... He gets his drums from somewhere. If you have to use a cc processors, you may want to think about contacting drum manufactures directly. Factory shipping and the margin is something substantial you can play with. Many product manufactures look at online sites as additional storefronts others are afrraid of cannibalizing existing sales.
As I would think that drums tend to be a "try before you buy" thing, you could address those objections by targeting advanced drummers and market geography that not's covered by retail.
Edited by dpyers