Shipping Integration (USPS, FedexG within US) CCSR: Conflicting claims by shopify, sendle, joovii

[rameshj]

Hello US based ship_integration Gurus:

• we ship our gourmet plantain chips withIN - 98% of which is sent by FedexG or USPS Parcel Select (we have on-line accounts for both to create labels, Click-N-Ship etc..) within 50 US states,
• based PRIMARILY on claims,  by shopify and eCommerce store_owners, of fewer seams in the N(ear)R(eal)T(ime) ship_rate calcs - we're transitioning from SSpace to shopify
• our products being very light, our dimensional weight tends to be high and ship_rate calc. and optimization has become vital to customer-retention
• As my first iteration, I would like to implement a shipping_flow based on an algorithm like so:
  • if destination is NOT in one of the 50 US States, apologize with a message and do not take an order,
  • if destination <= 10 miles of zip 78xxx, suggest curb_side pick-up at one of N locations,
  • elseif product weight UNDER 4 lb (our carton + packing material of 1 extra lb), use USPS Parcel Select+sendle - display min(shopify_rate, CCSR_USPS, sendle_rate)
  • else display CCSR_FedexG (using our negotiated Dimensional Weight denominator) 

After hours of back-n-forth's with CustSvc folks @ shopify, sendle , waiting in vain for response from https://joovii.com (even the website is often unresponsive), reading reviews at forums, https://apps.shopify.com/sendleshipping etc.., it appears that intra-US NRT ship_rate calcs. (and subsequent display of BEST rate to a prospective customer) is at best black-magic