Sending items from UK to the US - advice needed please

So I have the whole tariff thing sorted, but here's a new one. I have just been packing and buying a postage label to send an item to the US, and I discovered that Royal Mail (you knew it had to be them, right?) apparently no longer offers a service for obtaining a signature upon delivery if whatever you're sending weighs over 750 grams - all you get is an "electronic proof of delivery", which doesn't strike me as offering the same level of protection as an actual signature. Can any of my fellow Brits recommend a reputable courier that they've used which obtains a signature upon delivery? Thanks in advance!