Hybrid, weighted by town size. Post boxes only to the two small towns where the single shop might not have what you need, and buy in the three larger ones.
The budget angle usually favours buying as you go, because posting adds real cost per box and you buy in bulk at home which then sits in a box for three weeks. But the failure mode of buying is arriving at a village shop with three shelves and a dietary constraint, which is a genuinely bad day.
Being tied to opening hours is the thing people underestimate. Check whether each collection point holds parcels at weekends before you commit, because a box you cannot collect until Monday morning costs you a day.