To pay for your store, employees and such a 10 buck part would retail for 20.
This markup system is at least a century old.
2001 Isuzu Rodeo was recently serivced. Parts prices seemed high. $90 for spark plug wires, $54 for serpentine belt, $70 for valve cover gasket, etc. This is in addition to labor to install. What kind of markup is typical for parts, compared to retail?

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