Are you using a phone or mobile device?  If so, the system automatically sends you the details of your purchase with a Pay Now button.  It looks like an invoice, in that it shows what you bought, the postage cost and the total.  There is no option to continue shopping and allow the seller to combine with the mobile app, it has been a source of angst for many for this reason, it requires the buyer to pay immediately even if they are still shopping with the same seller. 

Your seller is probably telling you the truth, most sellers these days don't send invoices, they have no need to as buyers either choose to pay immediately, or if using the app, are left with no choice.

I think the shopping cart works the same way.  I know I purchased three items from the same seller yesterday using the cart, and had to pay for all three postage costs individually (luckily in this case it would not have been cheaper combined anyway).