How to buy more ad space on several websites?

This is simple. Under advertising you see the Advertising at banner. Click on that.

If you have not select any of the websites, then you need to do that first. Select one, and select Add to basket. Do that with as many websites as you want to advertise that time period on.

Then select with the last selection go to secure checkout. (Or click in top right on the shopping cart)

That is it. You are all set to go. (But be sure you already have send us here a copy of the banner and your website that you want to link !)