I detailed one approach to a similar problem here: https://community.ebay.com.au/t5/Selling/postage-costs-of-bulk-buying/m-p/2159632#M171011
You can also have multiple listings for the same item if each listing meets a different customer need - various quantities (as long as there is a significant difference between them) are allowed where otherwise they would be considered duplicate listings. I do this with some of my listings, where I have one listing for 10 of something, another listing for 25, and mention in each listing that there's other quantities available in store. The only thing I don't like about that method is quite often people will find the small quantity listing and buy multiples, equal to or even greater than the higher quantity listing (which provides a small discount). I tend to add extras in those cases, but I often also have to shift quantities over.
Other items I create variations for quantity, so buyers can choose between packs of something like 10, 25 and 50. Some of my items I offer in smaller quantities in a variation listing, and I get people mixing and matching between them to get the exact quantity they need. I have a flat rate postage model, though, which I find easier to manage than working out P&H on a per-order basis and means my buyers can just purchase and pay for any number of items without waiting for an invoice etc. Often times I pay significantly more for postage than my buyer has, but pretty much because they've bought a bulk quantity so I consider it a way of providing bulk purchase discounts. (This has its own nuances, or issues, to consider - I frequently have to explain why I can't provide the same low, flat rate postage when I do a custom bulk order and apply a price discount, because my postage rates already provide it as standard; i.e. they can have one or the other, and savings above store prices don't usually kick in until they've bought a really high quantity).
