you just don't use the ebay satchel / box promo codes that have set rates.

You need to put in your dimensions etc, I usually just put in promo code to get the info for weight / length etc quickly up and then change to own packaging and delete code, the dimensions are retained.

You can not process a batch of promo code satchels with a batch of regular post.  They must be done in seperate batches.

Can work out cheaper too, for example if I send a parcel within most (not all, but nearly) of Victoria up to 22kg - it only ever costs me $8.95

So savings to be had too.