Yes, you can use your own boxes as we’re all about flexibility! However, for quality and safety reasons we prefer you to use ours. They are heavy duty and designed to hold our security seals in place. If your things are already packed there’s probably no need to repack them. You would need to make sure your boxes are sufficiently sturdy to do the job (double-walled cardboard), not oversized, have good shape to them and are not heavier than 25kg when full. They must not be supermarket boxes or cheapies from the hardware store.
If you do choose to use your own box and it comes apart in transit, then by necessity we would need to repackage into one of our boxes. This would compromise our security protocols. And you may be charged for the materials and packing service.