Shopify App Developer - Convert Shipping Rates to Shipping Dates

I want a shopify app so I can create custom shipping rates in Shopify. I have tried using other apps with calendars but they don't meet my needs. I want to manipulate the shipping rates function so that: 1. I can turn the rates into a delivery day, showing the next 28 days available for delivery. Ideally using a calendar selection. Otherwise a list in date order. 2. After 3pm each day, tomorrow disappears and a future day in 28 days is added. (3pm being the time I will no longer accept orders for next day delivery). 3. In the back end, a menu to block out non-delivery days (Holidays, Sundays etc). So customers can't book one of those days. ALso in the back end to amend the cut off time (2) 4. The delivery date selection is added to order notes. 5. I can set variable rates. E.g. if the cart value is under £7.50 delivery = £1. If the cart value is over £7.50, delivery is free. Nice to Have - 1. Shipping dates variable to product availability. If the product is in stock, it can be shipped for tomorrow. If the product is out of stock, it could still sell but present a future delivery date (e.g. in 5 days time). Why use Shipping Rates for dates? - I am using other apps that mean I can't easily use existing date-selection apps - Existing date-selection apps override Shipping Rate selections (jumps to payment) and don't communicate the selected date back very well. - My Shopify website feeds onto iOS and Android Apps, so any function on my website gets lost on the app.. However Shipping rates feed through to the app. - There's already a lot of features on my Cart page. I'd prefer to keep this page clean and tidy. I think date selection should be on the delivery page. I've attached a few ideas. In terms of cost, I'd be interested to discuss a few options. For instance I'm happy for my idea to be shared and publicly available on the shopify app store - similar apps earn $9.99 per month.