Also just to add more clarity. The bulk quantities are specific quantities, not ranges.
For example, that bulk quantities are packs of 30 and 75 only. Think of it like this: you could buy 1 pen or you would buy the prepacked set of 10 pens or you could buy the extra large pack of 30. There are no in-between quantities. But you could buy 5 of the 10 packs or 2 of the 30 packs.
In that case, you need product variants. Seems like quantities or volume discount has nothing to do with what you are looking for since the quantities and discounts are baked into one sku. Here's an example product I set up that does what you are asking for. http://shop.bccf.ca:8001/catalogue/pen_395/