I want to create a recurring payment but instead of a standard monthly plan, I want users to add a selection of ecommerce items to a cart then when they checkout it charges them the same amount as the cart total each month.
The difference here is each users cart total will be different amounts so the plans solution wouldn't work.
Who is your payment processor ? (payment gateway you use) ?
may I ask what country you are operating your business from? depending on the country their may be some payment processors who have flexibility to accommodate your requirement. Solution I am suggesting is to use custom payment gateway integration to your Shopify store with ability to take recurring payments through the gateway. Some gateway's work with Gateway Tokenization.
so we can use Gateway Tokenization to create a custom gateway with your requirement.
I am not sure it helps.