Our website handles gift voucher sales for any other kind of business and we want to enable our codes to be used on the Shopify websites of our registered businesses.
I had some discussions with Support who sent us on documentation and information on building an App.
Our developer has asked that rather than building an App could we build into API calls?
Any help on this would be very much appreciated!
Thanks in advance
Like @Jayvin said, you will need an app to consume Shopify's APIs. That app will be issuing the requests.
There are different kind of apps, maybe a custom or a private app better fits your needs? Here is a page that explains those different app types https://help.shopify.com/en/manual/apps/app-types.