[mothered]

Is it okay if I say DNA using a fake sig on an order up to £250 for Zalando? (Clothing only)

Definitely.

Due to the cost and the method used (DNA), most likely an Investigation will be opened, whereby they'll cross-check with the carrier's consignment. So be prepared for that.

And for Nike, if the order is up to £200, what excuse should.i give it shoes will be in the parcel?

You can use the "wrong Item received" method, by purchasing a "stock" Item that's significantly cheaper. When they request the return, send that.