A $59 script is not a $59 project. Almost nobody budgets for the rest, and that is where the disappointment comes from rather than from the script itself.
Hosting that can actually run it
If it needs a specific PHP version, a queue worker or more memory than basic shared hosting gives, that is a monthly cost. Check the requirements against your current plan before you buy, not after.
Installation, if you are not doing it yourself
Author support covers questions about the documented steps. It does not cover somebody logging into your server and doing it for you. If you need that, price it.
The things it connects to
Payment gateways, SMS, mapping, email delivery. The script talks to them; it does not pay for them. A map on every page can quietly become the largest line in the budget.
Customisation you will want in month two
Almost everybody wants something changed once it is running. Either your own time or a developer's, and it is far cheaper to assume it now than to discover it later.
Support renewal
Author support runs for a set period. If you expect to still be asking questions in a year, extending it is a real cost, and a cheap one next to being stuck.
The useful comparison
Not script A at $39 against script B at $79. It is total first-year cost against what building it yourself would take. On that comparison, the licence price is usually noise.
