Repair businesses should always have commonly-needed replacement parts on hand. If a new customer asks for a repair and you respond, "I'll have it fixed in 5 to 7 business days," that customer will look elsewhere. Keep enough parts in supply for 1-2 weeks of common repair. Then, restock parts all at once to save on shipping costs.
Note: be sure that your inventory value isn't rotting away on the shelf. Batteries, for example, lose their maximum potential charge over time.