How much did they pay?
Your project has a recommended donation of $2. If someone comes in and sees that, they may only pay the minimum, thinking they're buying the project, and then not be able to access the files they're interested in because they're not included with your project; instead, they're files with a price tier.
If they did indeed pay $5 or more (excluding VAT) and can't access the files, they should contact support.