Oh really!? Wanna elaborate?
Sure. The info I'm posting is from my experience with Autozone's commercial sales division. It does not
exactly transfer over to other parts stores, but they all operate this way - it's just a matter of to what extent. (Of course there are stores like Parts Plus or Factory Motor Parts that only deal with commercial accounts, so not all of this applies to them.)
There are many prices for a single part at any parts store with a commercial division. From high to low, there's list, DIY, tiered, and cost.
List is a number that corporate pulls out of their ass. Nobody is actually charged the list price. List is designed to allow the shop to charge the customer "list price" for parts, allowing the shop to pocket the difference - and to make the shop feel all warm and fuzzy thinking they're getting an awesome discount. At Autozone, list price is almost always exactly twice tier 5 pricing.
DIY also known as Walk-In price is the price for Joe Snuffy that just walks in the door every other month for a part. It is usually significantly lower than list, but higher than most tiered pricing.
Tiered pricing is the pricing structure for commercial accounts, which varies based on sales over time. If shop A averages $50 a week of sales with the parts store, they only qualify for tier 1. Shop B at $500 a week qualifies for tier 3, and shop C at $2000/wk qualifies for tier 5. Tiered pricing is a discount to DIY, and the discount varies based on the markup (from cost to DIY price) of the item.
Cost is simply what it cost the parts house to purchase the part from their suppliers (Johnson Controls, Honeywell, Valeo, etc.)
So to put it all together, from the bottom up. For a low markup item, one that the parts store does not make that much profit on, like a cheap stock replacement air filter, for example:
Cost.....$6.73
Tier 5...$8.99
Tier 3...$9.49
Tier 1...$9.99
DIY......$9.99
List.....$17.98
And another example of a high markup item, where the parts store really makes their money, like a 65 series battery:
Cost....$28.73
Tier 5..$71.99(20% off DIY)
Tier 3..$80.99(10% off DIY)
Tier 1..$85.49(5% off DIY)
DIY.....$89.99
List....$143.98
On items that are very low markup, there is NO discount for commercial accounts, they pay the same as a walk in customer would pay. The only benefit is that the shop still gets free delivery.