Normally I'd suggest rate tables may be able to solve this, but it might be a little difficult when a much higher quantity is involved before the next weight bracket kicks in (I believe the rate tables deal in 1kg increments, though it may be possible to assign say 100g per item so it doesn't kick over to the next bracket until 11 items - more info on rate tables here, anyway: https://sellercentre.ebay.com.au/postage-rates-tables )
The suitability of alternatives would depend on the common quantities purchased. eg If you found a lot of people bought 10, then it might be worthwhile create a listing for a quantity of 10 and then just have something $10 P&H per lot of 10, of maybe a small discount if you can provide it.
Or, you could provide a flat rate of postage, and count on a few other things to cover additional postage costs (for example, if ten different people buy one each, you'd pay a 30c PayPal flagfall on each one. If someone buys ten, you'd only pay a 30c flagfall for all ten, so that's $2.70 extra you wouldn't otherwise have you could put towards additional postage costs. Heavier items may be priced slightly higher so that in a bulk purchase, everything is still covered. eg if you need to make sure a purchase of 11 items will give you the extra $10 for postage, the individual item price could include an extra 65c, if that makes sense.
i.e. 11 items would mean 10 x 30c PayPal flagfall you don't have to pay, so that's an extra $3 already, so then you just need to cover $7, which between 11 items would be around 63c each, obviously in some cases you'd wind up with a bit extra than you may have charged in other scenarios, (purchases of 10 items, or 20 items) but you can value add to those orders if you want to (signature on delivery service, insurance, or even express).
This is just an example, but it's another way to approach this kind of problem.
