That is a ridiculous response. It is impossible to know the size and
weight of the package until

1) The buyer has completed all purchases

2) The buyer has expressed their wishes, in terms of packaging and
delivery dates - SD labels expire!

3) The buyer has had the opportunity to switch to Collect once they
realise you are nearby

4) The buyer has the opportunity to request expedited delivery, which SD
does not offer (Tracked 24 does not offer a delivery guarantee)

5) An overseas buyer asks for a parcel to go to to UK address to save
costs - very common occurence

I could go on, but what EBAY needs to so is to review feedback from sellers

SD can only work if the seller is able to cancel the label immediately,
in order to offer a proper Customer Service Experience