The few figures I have seen on ebook returns are incredibly low, 1 percent or less. Even 5-10% would be reasonable. Amazon may be on the lookout for serial returners, but I have no doubt that its return policy is, in general, driving more sales than returns.
An exception to the low return rates might be poorly formatted ebooks. Another might be very short ebooks, although $0.99 for short content is reasonable if you know ahead of time how short it is.