I have an existing website with shopping cart features built in, and payment via Stripe or Paypal. Changes recently made by another freelancer include look up and calculation of postage charges through 2 different courier companies as well as Australia Post, then using a rounding method, choosing the cheapest postage charge. This functionality is not working perfectly and needs to be reviewed, as well as issues with the site at the moment where payment by credit card is returning a 'fatal system error'. Job will entail modification of existing code to look up postage charges (including possible screen scrape from existing couriers), as well as examination of code to establish cause for system errors. Further details to be provided once quote accepted.