Proof of payment seems to be not enough to judge that a company is not a scam. Within the site itself you can tell that they are a scam one. If the site does have a lot of ads placed and they are asking for an upfront payment. There can also be companies that are asking for an upfront payment that are legit. Some of them are the investing sites. Tough I haven't tried them yet but a lot of my online friends are earning from those investments sites.