Plastic roof sheets?

73yr old Pensioner: Door caller so got him to look at the plastic roofing on the garage which I use as an art studion.
Easy to get at and remove sheets which he says as water droplets are in between the walls of the sheets it need replacing before it leaks into the studion. £400 to replace four sheets, estimate on headed paper no main phone or address just a mobile and hand written [badly ] estimate with an 'Adendum' at the end that the cost may go up by max of £125 if any timbers need replacing. A friend tested rafters, as I can't climb ladders, and said timbers were sound so I think this may be a 'Let's stitch up the old boy he looks like he has plenty'. I was a builder and prices have risen since I did the job but £400 , and he got a bit stroppy because I didn't agree straightaway.
At this moment in time I've shelved it as it's not leaking but he said it would.

opinions please! thanks Ian