Refund Policy
"We will maintain a reasonable policy regarding requests for refunds. We reserve the right to refuse a refund or to apply a partial refund. An administration charge may be applied where a refund is offered."
You can see this within terms and conditions on each add-on but I would believe once you download an add-on it probably be really hard to get a refund but this is something you will likely need to wait for nix to reply about.
Bugs happens with all add-ons and nix is really quick to fix them!