I have allocated half a day for this i.e. 4 hours, this is long enough to take a copy of your site, install it on our local server, find the issue and make the necessary changes to the relevant .php file to make it work; of course then all we will have to do it upload the changed file(s) in order to have it working on your server.
The altered files will have comments inside to document the changes and I will provide a change log for your records, so that you know what we have done for future reference.
The total lead-time for this project is 2 days to allow for correspondence with the vendor should we need to speak with them.
Kind regards,
Darren