This is just crazy, an XJ650 sump drain plug ally washer can be ordered from Wemoto for £0.48 with zero P&P cost. A second class stamp costs 65p.
Motorworks sell the full range of crush washers & O ring for a Camhead RT. The engine drain crush washer is £0.36 plus £3.90 P&P = £4.26.
Wemoto did have the same RT crush washer listed for £0.50, although not available for immediate delivery. This is an OEM part sourced from BMW Motorrad.
I've sent Wemoto the full list of four crush washers & one O ring for a Camhead RT. They are going to look into price & availability.
If the total cost is £4.99 or less, the P&P will be zero. Even something like an oil filter, (if less than £4.99) would be free P&P.
Locally, MSG Bike Gear do a "Price Match" and their prices include free P&P.