Sigh.
The old P&P argument.
If you have a load of products of wildly differing size and weight, then to automate a formula to calculate pricing is an absolute nightmare. You would need to factor in volumetric sizing, area codes etc. Customers in Highlands & Islands and Norn Iron will complain that they get charged so much more, people will complain that they don't know the pricing up front. You can't please everyone, you've just got to try and do your best.
A lot of stores like me take the decision to go for a flat charge, this means that sometimes the customer pays more than the cost and sometime less. I doubt very much that in the long term I end 'up' on the deal.
Other stores go 'free' postage but charge a higher price for the product in the first instance, good for the buyer of a single low value item but not so good for a multiple item order or more expensive item. Again sometimes the buyer is the up, sometimes not.
Franking machines are a little cheaper for postage but the hire firm insists you use their ink & labels (it's in the contract and they know how many impression you have made). The end result is it's slightly cheaper but not as much as you might suppose, the biggest gain is the convenience.