Help writing a shopify app

we are a courier company, currently, we use custom api keys for every customer wanting to syn with us and Shopify. we need to get this rebuilt into a public published app. we need to pull a shopify customer's orders, order details, address etc then once we generate a freight ticket push back it is fulfilled and the label number. we do all this now but on individual api keys. this needs to be a app. we need this built by someone who has had apps published before.

we have tried in the past but struggled to get past the publishing rules