Yep, anything that looks like a bag is fine, including reused courier bags, thicker carrier bags, card or paper envelopes or bags (if strong enough for item & item won't get damaged if left in the rain). Can always fold down a bag to fit the paid dimensions too with sticky tape.For larger bags, look at Pass the Parcel, who have a more generous weight limit & no thickness limit (10cm on TM Book a Courier).