I used to buy from Sideshow on their payment plans before I knew about other places to buy from, but now I buy locally with a $100 deposit for figures I can wait for and directly from HK for figures I don't want to wait for.
I have a separate digital piggybank for figures I have pre-ordered. I don't pay them in full in advance, only an NRD, but I can pay them at every moment should it be necessary. If I can't do that because of other bills, I wouldn't buy the figure.
The reason I don't pay them fully in advance is because I don't like to pay something up front that most likely is 2+years away. (Especially in Europe). I don't want to run the risk in paying everything, only to have the shop go bankrupt or something. Then my money will be gone. Even creditcard insurances and everything often don't cover 2+ years, so I'm not paying full in advance.
I'm not adverse to paying off over time, but generally I just put in the whole cost of a figure in my budget (my budget goes until end of 2022) for when I expect them to come out.