"Sending multiple items together in one package is a basic function of online retail everywhere else. Goodness knows why eBay haven't updated their GSP to incorporate this fundamental feature.
Do they even have any plans to fix this?"
No because eBay is not an online retailer - they don't sell anything but services from which they collect fees. The higher the transaction costs the more fees they take. Not just final value fees plus postage - but fees on taxes plus currency transaction fees on the total the buyer pays.
It is not just GSP where this problem occurs - combined postage to international buyers is also problematic with eBay often showing the buyer a banner that the seller doesn't combine postage in order to dissuade buyers from requesting combined postage. Many buyers just give up and go elsewhere.
I have had a number of regular international buyers who frequently bought multiple items at once now tell me they buy from other platforms where total costs work out cheaper. The buyers I have retained know to request a 'Buy it Now' with a bundle of the items they want. More work but it retains the sale; nevertheless it shouldn't be this way.
eBay are only interested in a quick dollar today but don't seem to realise this increases the cost to both buyer and seller, damages the buyer experience, and drives buyers away. No thought is apparently given to retaining buyers or encouraging them to make multiple purchases.
