His store is not working, better write your own from scratch as you need it on PHP to make calls to wallet/node API, you can take some API calls as examples, but it is almost useless here. If you will find PHP devs who will do it or want to create bounty through Grin CC (even better), let us now, we can discuss Terms of Reference together 
Also check, you are not alone, we have Node.js/React developer here with same ideas: Grin Payment Service