This is a good example of customers thinking they can save a few bob by getting the gear themselves. I would suggest if you supplied labour only then you do not do any remedial work FOC.
Customers complain when we charge more for material than they can get it for, and this is a classic example of where it will come back and bite them. If you had supplied material and added a mark up as you should do
you would contact the supplier and sort out the problem between you FOC for the customer. However, now the customer has to sort out new material you are not being paid.
I have done labour only a few times (plumbing and heating) as the customer wants to save cash but it was 100% clear from the outset that all I was liable for was labour and workmanship, and if called back to a problem i would call back and inspect and if my fault i would fix FOC.
If however a revist was due to faulty material then I expect the customer to sort it with the supplier, and I would invoice £60 call out and subsequent visit to repair.
An example was a B&Q bathroom suite which had a leaking bath trap, about 6 weeks after the install.I got a call saying there was water coming out of the kitchen light,I visited, checked it out and found the trap thread stripped,( i had measured up the job and they bought all the material and had bought all crappy waste fittings) told them they needed a new trap, and suggested a better quality one.
I prefer McAlpine and suggested i go and get one and renew it on the first visit for the £60 plus cost of trap, (cause i'm so nice) and they could take up the problem with the supplier, if not i wanted my £60 anyway so the guy said ok he grumbled a bit that the £60 well wiped out the savings he made by supplying, i said correctomundo, and if he had any other prob with the cheap stuff he bought then he would have the same problem again.
In effect he would have been better letting me buy the stuff as suggested, i had to laugh when i thought back as i said i would supply all the stuff for £50, he said no he would supply, superb, what people forget is that when we supply we take responsibility for the gear as well, so we end up fighting with suppliers and the customer gets someone to fix for nothing.
We are absolutely correct putting something on material purchase for warranty and purchasing hassle