I take it you are referring to using eBay's checkout in which case just set up your preferences there to send out the 'you have won' email which includes a pay button.
I find most pay by this route and so I only have to send out invoices specifically to international and multiple item buyers as I do not find the automatic postage discounting system matches my wares very well.
If you are finding 20 tedious just wait until you become a big success and sell to 100's a week !