You can use your own stamps, if you trigger a Simple Delivery label and not use it,  the refunds goes back to whoever paid for the label, and it takes 30 days to be refunded,  see here.....

You also, cannot now add any new tracking,   eBay suggest sending the buyer a message with those tracking details.  If a case is opened later for item not received,   you will be able to add those tracking details.

A refund is sent to whoever paid for the SD label:

• If you selected 'Buyer pays’ when you created the listing, your buyer will automatically be refunded for any Simple Delivery label that isn’t used after 30 days

• If you selected ‘Seller pays’ and offered free postage on your listing, you'll need to fill out the Request a refund for Simple Delivery form for the label that wasn’t used. Make sure to send your request within 14 days of your item selling.

Payouts for those items will be held for a minimum of 14 days.

@freckoid 

Adding to good advice received for how to ignore Simple Delivery. Here is how to set up Custom Delivery from the get-go. Be warned, it is glitchy today.

When it's working normally Custom Postage is generally available for items £20 and under. To activate CP within your listing when Simple Delivery is auto selected you choose 'Large Letter'. This should activate the CP tile. Click this, now you can enter any courier/size/weight you like. It doesn't have to be Large Letter or under 100g anymore - it can be smaller Letter or even a Parcel.

Most of your items are Royal Mail Small Parcel size, so now I am going to tell you why you shouldn't use stamps when you choose CP. Do you know that when you send 2nd Class Small Parcel your proof-of-posting receipt has a Reference Number on it? This is RM's internal tracking number. It works in eBay, and you can enter it as tracking there. If you absolutely must use stamps, put at least 1p short and pay the difference at the Post Office - have them sell you 1p with franked postage (not a stamp) so you will still receive a Reference Number. Although they won't thank you for the extra work! That way you have some tracking protection. Good luck!