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Business acquires materials, combines them with labour, total cost of job is labour plus materials plus a premium on those materials. That quite comfortably describes both a bike shop and the operation of a tradesperson we’re talking about.
The key difference is that with most building jobs you have variations during the period that the service is being provided.
If everything is known upfront and a fixed price is agreed, it makes not a jot of difference how the cost is broken down into labour and materials. The hard bit comes when you are pricing changes - that's where you might get a bit miffed if you found that there was "hidden profit" in what you thought was a straight recharge for materials.
I'm coming at this from the perspective of working in a highly regulated industry. If I ring up a counterparty and say "do you want this bond, I paid £100 for it", and in fact I paid £95, then I could get absolutely reamed by my regulator.
Business acquires materials, combines them with labour, total cost of job is labour plus materials plus a premium on those materials.
That quite comfortably describes both a bike shop and the operation of a tradesperson we’re talking about.
Amusingly, it’s also fairly accepted practice with bike shops that if you provide the materials yourself the labour actually costs extra!