The dimension of classical rehearsal packaging is to precisely weigh your items. Recently, premium agents started combining it with a consultant service.
First of all, the rehearsal packaging service itself might cost you a couple dollars, but it does not force you to place the order afterwards.
Superbuy, Wegobuy, Sugargoo and presumably all other agents apply the rehearsal packaging on a shipping order level, not on item level. That means you must have already chosen a shipping service and the reason you're getting rehearsal packaging in the first place is a lowered shipping price on your parcel as a whole, not updated/decreased weight of individual items.
Wegobuy offers a method to decrease the weight of items in your warehouse, but you still have to lock down a shipping service, so it ends up being the same, only more risky.
If you decide to cancel the parcel the rehearsal packaging has been applied to and go with a different shipping service, that offer is lost and there's no concrete evidence it makes your new choice cheaper. After 48 hours, if you haven't accepted, the offer expires.
If you accept you get (almost) fixed, cheaper prices.