Welcome to the fold!
I find that people either know exactly what quilt pattern or fabric they want, or they have only a vague idea about colors but no concrete idea of how to find it. I have never gone shopping in person with a client (no objection to doing it, just never worked out that way). For those people who have a vague idea, I keep sending them links to things I think would be appropriate until they see something they like. For people with concrete ideas, I send them to major online fabric retailers (Hancock's of Paducah, for example) and suggest that they browse, set up a wishlist, and email it to me.
I charge a fixed amount for the work, and then add the specific cost of the materials for the project. If I use stash materials I use an average estimate of the cost, which gets tricky because most of my stash was bought over the years, and on sale, and I really don't know what its current cost would be!
I am comfortable with my system, even though on those occasions when I have calculated the hours I have put into my work I rarely get more than about $3/hour...I just love doing it! I have never had any problem with people not reimbursing me, but I have started writing out formal contracts as an aide-memoire - amazing how many details you forget when you don't start a project until several months after you discuss it with your client!
I am glad that you have found a good market so quickly. It is a lovely confidence booster to have people eager to buy your work.
Alison