You can use stamps - with some caveats.

- You will still be charged for the Simple Delivery label 

- Within 14 days of despatch - you need to fill out the form here to request a refund of the postage you didn't use (I swear it used to state you had to use a tracked method still - but can't find that anymore)

- There is no way to add alternative tracking on or confirm despatch.

- As such, you will have to wait about 14 days for payment (past the last estimated delivery date)

- Note with stamps you may not have any proof of delivery - so its worth noting that if its a reasonably valuable order

Good advice from @jonatjonatjonat 

I can only see one of your items with free postage/seller pays and that is a fairly pricey tracksuit.

All I will add is that if the item is sent as a parcel at the Post Office, you will get proof of posting and confirmation of delivery even if you use stamps.

However, it would only be covered for loss/damage for up to £20.