I had a buyer tell me the same thing last night.  Trying to buy 2 books off my listings, items in cart & no "request total" button anywhere in sight - also using a PC.

Luckily I was able to change the postage amounts to total the combined postage she needed to pay & away we went. 

However, I'm left wondering how many buyers don't get the icon & just decide it's all to hard to buy multiples with combined postage. I'm thinking a lot of sales missed!!