What economics theory explains changes in purchasing different types of goods?
If you research "discount rates", "time preference" or similar, you'll find all sorts of stuff about seemingly inconsistent decisions, some of which can in fact be explained quite well and some of which show some ways that people can be dumb sometimes. You'll find lots of approaches to modelling comparisons involving things like someone revealing an implicit interest rate of over 100% per annum in one situation while accepting 5% as a good return in another situation. 

I wouldn't focus too much on any specific paper. Check the bibliographies and see the variety of what's out there, and maybe narrow in on a handful of most-promising conceptualizations afterwards.

As a point of contrast, you could introduce the lifetime income hypothesis of Friedman as discussed in many 1st and 2nd year textbooks.