Change shipping method

Hi @basvweb!

Thank you for reaching out.

I think you could achieve this using an extension like Table Rate Shipping.

First, you can set different shipping classes, one for small products that fit through a letterbox and one for products that do not fit through it.

Then, you could apply multiple rules based on quantity to the shipping class you select. So if a customer buys more than one product (or the number you desire) from the same shipping class, it will be charged more for shipping.

Check more about it here:
https://woocommerce.com/document/table-rate-shipping/#applying-multiple-rules-to-the-same-shipping-class

If this extension gets your attention, you can try it and see if it fits you; we have a 30-day money-back guarantee if you are unhappy with the outcome. Check our Refund Policy here.

If you have any pre-sales questions about it, kindly contact us at https://woocommerce.com/my-account/create-a-ticket/?form=ticket. You may need to create an account before you can access that page.

I hope this helps!