If you lost the box from your original refill order, you can request a new label through the Collect+ site. and use any box to send back your empties.
Just make sure to please write your customer number on the Collect+ label in "Ref Box". Your customer number can be found in your account under "Order History".
You can also request a new label through InPost and use any box to send back your empties.
Once you enter your information in the InPost link, you will be sent a unique QR code. Take your parcel to the nearest locker and scan your QR code at the locker.