Hello 'chappy',
It's time consuming to read a whole listing, that's true. But it is also a really good idea to do it anyway. So many problems people have are because they didn't read the whole listing.
A lot of listings that say 'wait for invoice' can be paid the instant you win or buy. If a shipping fee is included and you only want the one thing, you can pay it immediately. If no shipping fee is included, - I'll assume you inquired about the fee with the seller beforehand.
Most times it makes no difference so I'm not sure why some sellers have the 'wait' request. On occasion though, the invoice may show a couple bucks off the shipping. Bonus! And every now and again some clever sod sends an invoice with additional insurance and handling fees not stated in the listing, -- and not allowed on ebay Canada. Other times they want you to wait so they can add the tax.
But if you're not ordering multiples, and the price + $hipping appears in your Purchase History, pay it if you want to. Who knows, maybe some sellers feel they are controlling the transaction by making you wait for the invoice. There's all kinds of people in the world.