Five rules, and each makes a different trade.
Free
Simple, and attractive to buyers. You are paying for it out of the sale, and it comes off your balance as a deduction rather than hiding inside the price.
Works when your items are small and uniform. Hurts badly when a heavy item sells cheap.
Flat
One price per order. Easy to understand, easy to get wrong in both directions — you lose on the heavy ones and overcharge on the light ones.
By weight
Slabs: up to 500g costs this, up to 1kg costs that, plus a rate per extra 500g. The fairest and the fiddliest.
Important: couriers bill on chargeable weight, which is the greater of the real weight and the space the box occupies. A big light box is billed as heavy. Set your slabs against what the courier actually charges, not what the item weighs on your scales.
By order value
Cheap delivery on expensive orders. A way of absorbing cost where you can afford it.
Free above a threshold
The one that changes behaviour: it makes a second item nearly free to the buyer and so raises what they spend.
The thing to do first
Get real courier numbers for your three most common parcel sizes to three typical distances. Set rates against those. Everything above is guesswork until you have them.
Questions people ask
Should I absorb delivery?
Sometimes it wins more than it costs. Work it out across a show rather than per item.
Why is a light item expensive to post?
Volumetric weight. The box takes the space of something heavy.
Do I pay commission on delivery I collect?
Your fee screen states exactly what the commission is calculated on. Check there rather than assuming.
Can rates differ per zone?
Yes, and they should.
Next: offering pickup.