I knew Digikey had the £33 (> gets free delivery) arrangement (It use to be £50 (> gets free delivery) for a long while, if I remember correctly), but did not realise Mouser did it as well. I guess it was competition and one started the trend and the other(s), copied it. Digikey also seem to deliver quickly as well.
Sorry my original answer missed it, but I did not know Mouser had the same policy.
I find it a real pain, when the delivery company charges extra brokerage fees and hassles me for extra fees. With foreign parcels, so I really like it when the electronics supplier handles it for me.