i figured this out after i posted (of course) but the solution is not very intuitive.
for anyone searching later on:
- on product page, set weight = 0, container = small tube, and capacity = 999
- on shipping settings, set domestic = 0, international = 1, small tube price = 0, & small tube ounces = 6
of course, there's no way to update all items with a capacity = 999 so it looks like it one by one time.