Why is it variable?

[Kathy]

Hello everyone, I'm a student and now I am doing problems on Managerial Accounting. In my textbook there's one problem that shows "Administrative Office Supplies (Billing)" that I dont understand what it really is. Is that Administrative Expense or Selling Expense (because of billing). The answer key says it is variable expense and doesn't mention whether it's administrative or selling cost.
Could you please help me clarify this kind of expense? and Why is it variable? Personally I think it's fixed expenses.
Thank you very much!
Here I attach the answer key photo.

 

[kirby]

If you send out one invoice in january and 10,000 invoices in february what will happen to your costs for postage, stationery and labor to mail that out? Still think its a fixed cost?