Paying over time one might be able to do and the interest is the cost of taking the time. All loans (what a credit card can be) have interest. If you want/need something "big" (house, furniture, appliances, car) you buy it over time; if you haven't gotten medical or dental insurance (and dental insurance might be a "wash" as what one has to pay each month for the insurance plus what one has to pay as one's portion of the dental work, plus large-scale dental work like crowns are not very covered and have caps/limits), it can be to one's advantage to just work out a payment plan with the dentist but if that isn't possible, charging it as a large expense and paying what one can afford is a legitimate road.

Interest only becomes a problem if you do not understand it, have too high an interest rate and/or pay too little in one's monthly payment to pay off the debt. But that's how most people pay for school (loans) and other big items and it sounds like billi's work she's having done is a big item. If one cannot get other, less expensive means of getting work done, paying over time can fit into one's budget easier sometimes than not paying rent/buying food at all, and getting behind in those and other areas/bills and then having to play catch-up. Money over time is a good leverage if you use it wisely. Interest works both ways, one invests money to make interest and one can buy time by paying interest.