Yes and no...

No because every time someone clicks 'add to cart', the cart page is loaded (which infuriates me, TBH).

Yes, if they're willing and able to use a cumbersome workaround. Sometimes, if I know I want to order several items, I open the listing in several tabs and select one per tab, closing them off as I go until the last one has loaded with all items.

Variations within the same listing are treated as separate listings, whether they're in the cart or purchased by just clicking 'buy now' and confirming, so the "each additional item" type postage discount won't work on multiple purchases of different variations (for example 1 blue widget and 1 yellow widget will result in 2 lots of full postage, but 2 blue widgets would calculate 1 full postage, then add whatever was put into the field for additional items - maddening).

The only solution to that is to create a postage discount rule and apply it to that listing, that way the postage will should automatically calculate in the cart or after all purchases have been confirmed, usually without the need for a new invoice and no matter what quantity / variations are bought. (I crossed out will because it's eBay, and not everything works the way it's supposed to).