Some upholstery shops don't accept outside COM's as it cuts their margins way down on the job. It's like taking parts for your car that you bought off EBAY and asking your mechanic to install them.
Duane Collie
Straight answers from thirty-six years in the business.
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Most mechanics don't keep a vast inventory of parts, they diagnose the problem and then order. The traditional big upholstery shop is renting space enough for lots of fabric books, displays, and other inventory, and they have to make every square foot pay, as you know. So in this business model, the sunk costs make it difficult to do COM at a price the consumer will accept. It's a circular problem.
The small upholsterer working out of his home doesn't have these costs. Here in southern New Hampshire, the small vendors carry little or no inventory, e.g., the slipcover lady sends her customers to the local fabric shops that know her, and if there's a question, fields it with her cell. So that's the solution I'd suggest for the OP.