Hi,
It is usually best to have the sponsor pay the fee (to show that the sponsor is taking care of things financially for the person who is immigrating). You can pay it the same way you pay the sponsorship application fee. Payment online should be quickest; if you do, just print out the receipt and fax it to the visa office handling your application. (If you have the name of your visa officer there, you can send the receipt directly to him/her).