There are no special rules apart from, I name the EU countries in the postal set up. Europe, European Union, Ireland, Germany, France, Greece, Italy, Spain, Asia, Japan, Australia, Russian Federation.
My product has a unit cost of £23.99.
I subsidise the postal charge to EU ( I charge £6.95 for Royal Mail Tracked International.) and offer a discount on two or more. Buy 2 get 5% off, Buy 3 get 10% off, Buy 4 get 15% off. As I charge £6.95 regardless of where the order is sent, I take a bigger hit for countries further away. I do this as I am building sales to AUS and NZ.
So, a typical Invoice to Germany for two items would be, 2 X widgets, say £47.90, includes eBay added VAT rate at 7%. Subtotal: £47.98 Plus Postage (subsidised by my company, circa £2.00) £6.95 plus VAT (eBay collected) £3.69. Less Discount -£2.40. Total Charge to Customer £56.22