Lotta of close guesses here, but nobody really nailed it, so here's how it works.
All parts have a MSRP of course. They are SUGGESTED list price. Most dealers like us stick to those prices. Most dealers have the option in their store's business software to add a multiplier to every part they sell. 10%, 15%, 25%, whatever they choose to do. They'll come up with various reasons for doing this if challenged, but it is their choice to do that if they want. So, in most cases where you see one shop charging more than another this is what's going on. The guy just hits a key on the computer and every part in inventory goes up 15%.
Shipping costs: The way we do it is the only time we will add an incoming freight charge is when it is a special order that was generated specifically for that customer. Some dealers add freight to every over the counter parts sale; even for parts sitting on the shelf. Can you imagine doing that 50 times on an order that may not have even had a freight charge to the dealer? That's just a way to bang customers, and it's awful hard to defend something like that. You know how you get a phone bill or something and it has all those little fees that you don't even understand? Same thing with some of the ways dealers will add fees.
Husky and Stihl have different dealer policies that set price and freight discounts. Husky's is quite a bit more dealer friendly than Stihl's. Actual parts price discount for Husky is determined by a pre-season booking order. It has nothing to do with a dealer's wholegoods volume. (Not sure how Stihl does that.) We do a LOT of parts, so we get the best deal they offer. Participating in the program allows Husky dealers to get free shipping on most in-season parts orders for any order that totals $100. This is one of the best programs out there. Stihl makes you buy a lot more for free freight. But the big difference is that Husky allows accessories such as PPE, hand tools, bars and chain, etc to count as parts and it all rides together. Last I knew, Stihl makes you anti up separately for parts and accessories. Not to forget that Stihl bangs their dealers for 1% on every single invoice they cut as a "marketing fee", and you can start to understand why some dealers feel they have costs that need to be recovered somewhere.