Question

The cost of ordering multiple items is sometimes, always or never proportional

Answer 1

Unless there is no promotion like "buy 2 get 1 free" or "buy 2 get 1 for 50%" etc. then the cost of ordering multiple items is always proportional. Let's say something costs $5. Two items will cost $10, three $15, four $20 etc. These numbers are all proportional. Hopefully that's what you meant :)