First item + each additional (physical items only), with free-over threshold

{% liquid
  # Charge $5 for the first shippable item, $2 for each additional shippable item.
  # If cart subtotal >= $100, shipping is free.

  # --- editable settings ---
  assign service_name = "Incremental Shipping"
  assign service_code = "INCREMENTAL"
  assign currency = "USD"
  assign description = "First item + additional items"
  assign first_item_price = 500                 # cents
  assign additional_item_price = 200            # cents per extra item
  assign free_over_subtotal = 10000             # cents threshold for free shipping

  # --- logic ---
  assign subtotal = 0
  assign shippable_qty = 0

  for item in shopify_rate_check.items
    assign line_price = item.price | times: item.quantity
    assign subtotal = subtotal | plus: line_price

    if item.requires_shipping
      assign shippable_qty = shippable_qty | plus: item.quantity
    endif
  endfor

  if subtotal >= free_over_subtotal
    assign shipping_price = 0
  else
    assign extra_qty = shippable_qty | minus: 1
    if extra_qty < 0
      assign extra_qty = 0
    endif
    assign shipping_price = first_item_price | plus: (additional_item_price | times: extra_qty)
  endif

  assign rate = rate | merge: "currency", currency
  assign rate = rate | merge: "service_name", service_name
  assign rate = rate | merge: "service_code", service_code
  assign rate = rate | merge: "total_price", shipping_price
  assign rate = rate | merge: "description", description

  assign rates = rates | array_append: rate
%}
{
  "rates": {{ rates | array_wrap | json }}
}