I have had 2 cancellations in the last 4 weeks - both from buyers who completely underpaid by several hundred dollars. I got a third client like this just this morning - $5 paid for $275 worth of work.
In every case, I have sent a nice, friendly “please pay the extra amount required” (in a better worded way) with a custom extra. The first 2 requested cancellations which I granted, but the third, has just accepted the extra amount with a nice comment about how he was unsure as he had seen lots of really low pricing on the site.
My initial reaction to the third one was to write a not so friendly message and cancellation request. After a quick read-over, I figured that I was being too harsh so I deleted it and decided to go back to my usual benefit-of-the-doubt style of response. I guess the bright lights of Fiverr pricing sometimes dazzles people and they get confused or delirious about how much value is here.