@Sandra, the agent is taking a cut from your work, after a five minute chat with the customer, and then tries to butter you up to do it for peanuts.
A suggestion is to allow the agent to quote only for new zips, shortening skirts, dresses and trousers (pricing agreed in advance).
Anything else, they bring direct to you for a quote ( - eg- this will take several hours and cost this, including materials such as zips. Or- just too busy at present for such a long job)
A tailor in town posted alteration prices in the window to let folk see how much work costs.
He is still in business, and Primark is next door if they want to buy new (!) instead of fixing decent clothes.