It seems to me like it shouldn’t matter how often you buy the $15 items, technically. Even if you always bought $125 items and never bought $15 items, your heuristic still wouldn’t be completely irrational. If you only buy $125 items, you’ll only be able to buy 4% more stuff with your income, as compared to 33% more stuff if you always buy $15 items.
It seems to me like it shouldn’t matter how often you buy the $15 items, technically. Even if you always bought $125 items and never bought $15 items, your heuristic still wouldn’t be completely irrational. If you only buy $125 items, you’ll only be able to buy 4% more stuff with your income, as compared to 33% more stuff if you always buy $15 items.