How can I correctly charge customers for Saturday delivery on Shopify?

[Timk1]

I'm trying to work out a way of charging customers correctly for a Saturday delivery... because our courier charges more for this. 

I have a delivery date picker set up on the cart page. 

I have a Saturday shipping rate created that is based on a high cart weight threshold. The idea being that if I can artificially log the cart as heavy - the Saturday rate will apply.

(The actual shipping weight and process is managed outside of Shopify) 

I have some code that recognises a Saturday chosen on the cart page. 

I've tried 3 approaches 

1. If Saturday is chosen increase the {{ cart.total_weight }}
2. If Saturday is chosen add a Saturday delivery line item (through an AJAX  post) that has no value but does have a weight.

3. If Saturday is chosen increase the first line item weight.

All coded on the main-cart-items.liquid file on a Dawn theme store.

I can't get either approach to work and I'm beginning to wonder if Shopify does not allow this sort of work around? Shopify Plus is not an option (we are small) and I've got this far without signing up for an app that will do it so a bit reluctant to take that approach. 

I can post the code for both approaches if anyone wants to see, but a - yes or no you cant do this would be as helpful.

Thanks

[Timk1 · Accepted solution]

I got this to work (eventually) through the extra Saturday line item method. The POST wanted a variant ID not the overall product one. I created a variant and used that number and it worked. 

[nash_tech]

Hello Timk1!

That's a great way to solve this. If you would like to remove your reliance on one courier and expand your delivery options, please read on!

We developed an app that enables stores to offer same day and scheduled local delivery by outsourcing to fleets such as Doordash, Uber, Roadie, and more. If you have internal drivers you can manage them through the app as well and decide whether to fulfill items in-house or outsource them. We built a custom widget that you can add to your store to your customers choose from pre-defined delivery windows. You can check out the Nash Shopify app (https://apps.shopify.com/nash) and get in touch with the team at our website (https://www.usenash.com). We'll help you set up your account and help you expand your deliveries ASAP!