Shipping Rule: add calculation option for quantity

[casesolved-co-uk]

The three available options for shipping rule are:

• Fixed
• Net Total
• Net Weight

There are however situations where calculating shipping based on Quantity are desirable. For example:

• You can only ship an item individually, therefore the shipping charge needs to a fixed amount x quantity
• You might want to specify a shipping charge limit, so let's say your shipping is fixed at £2 per item and the limit is £10, so with each additional quantity: £2, £4, £6, £8, £10, £10, £10, £10
• You might want to specify a percentage discount on shipping for each additional item, so let's say 20%, so with each additional quantity: 0%, 20%, 40%, 60%, 80%, 100%, 100%, 100%, 100%. i.e. if you order quantity 6, your shipping is free.

Combining the above the calculation would be:

min(shipping x quantity x (1 - min(percentage x (quantity - 1), 1)), shipping_limit)