Develop Apps to buy parcel insurance through eBay, Shopify, Volusion, Stamps and Paypal

We want to offer a package insurance option either in their shopping cart or as a standalone app that draws shipping data from the respective architectures. We need to determine the best way of implementing this feature ....extension or app. This app would have to be integrated into the cart, with a customer prompt. The app would have to be enabled with a key, which would set the price tier. The insurance rates would have three tiers based on volume and risk. The decision template would look something like this, with sample values: CARRIER Value Tier 1 Price Tier 2 Price Tier3 Price Units (Value/$100) rounded up Premium (assume Tier 1 is activated) USPS Domestic $250 $0.75 $0.85 $1.00 3 $2.25 USPS Intl $250 $1.10 $1.25 $1.50 3 $3.30 UPS/FedEx Domestic $250 $0.35 $0.40 $0.60 3 $1.05 UPS/FedEx Int $2.50 $0.75 $0.85 $1.10 3 $2.25 Alternatively, we could employ a single price per $100 of coverage The app should be simple, automatically calculating the insurance cost and asking if the end customer wishes to buy insurance. If they do, the cost is added to the checkout balance. The app would have to access the carrier used, domestic or international and the value of the shipment. It would also have to transmit to us a manifest each week of what this customer shipped with insurance listing each insured parcel. Domestic or international , value and insurance cost. We would use this to bill the customer (deduct from a prepaid account using a credit card) and bind the coverage in our ERP. How many hours of work would this be?