I would agree with some of the other posters on your other thread that it's worth considering whether you are genuinely self-employed. Do you set your own hours, use your own equipment, get paid by results rather than by the hour, that type of thing?
Assuming you are genuinely self-employed it will depend on whether there is provision to make deductions of this type in your contract.
If there isn't, then it could be a breach of contract, however also check the termination provision and other clauses as they could terminate, or reduce or withhold work or whatever.
I have not yet received a copy of my service agreement, despite asking for it multiple times.
I do set my own hours, so long as parcels are delivered by a certain time, I can work as and when it suits me. I use my own car, but they provide my gizmo to scan parcel barcodes and to track me. I am paid by results, so I am paid per successful delivery.
I will check once I get my service agreement, which will hopefully be very soon.
Thanks for all of your answers. They have been very helpful.