When you give Sendle 'authority to leave', you are allowing Sendle to leave the parcel at the recipient's address without a signature when the parcel is delivered.
The benefit of delivering parcels with authority to leave is that the parcel will almost always be delivered on the first attempt. This means that the receiver doesn't have to arrange a redelivery or collect the parcel from a local collection point.
Sendle includes the option to get a free signature on delivery for all parcel sizes above 2kg. As 500g satchels typically fit into post boxes, you can not opt for a signature on delivery for this size. To add a signature, book your 500g satchels as a 2kg parcel.