The postage and packing costs charged by different companies are becoming a differentiator - increasingly more frequently than the parts costs. I'm sure it's not by coincidence that Canleys, Rimmers and Paddocks all run their online portals in a slightly different manner so VAT and delivery are added at different points in the transactions.
Then there's the question of whether or not the company tries to recover part of the wage costs of the person doing the picking and packing via the delivery charges. If the company is handling a lot of orders they can do so with only a small inflation in the p&p cost, but somewhere like the club shop is not going to be able to do that and remain competitive, although I suspect they benchmark against other online suppliers.
Gully