I don't know if you can use registered mail for parcels originating in Germany.
But the real fuckup is that it was sent via DHL. It got fucked in the handover from DHL to USPS (the delivery agent) somewhere in New Jersey.
Registered airmail with Deutsche Post, if even possible, would've cost a fortune. But then the box was worth a fortune, so...
You can send registered mail between most countries these days - many first world ones participate in a worldwide tracking system so you can actually track a package through borders. With other countries, you get a proof of delivery.
And what do you think the "D" in DHL stands for? Yes, DHL is headquartered in Germany, and in every continent outside of North America, is considered to be a top-tier delivery service. FedEx and UPS are considered second rate services.
Anyhow, if you're wrapping stuff for delivery, the postal service is quite good given the volume. (USPS handles more mail in 3 days than FedEx in a year, and in 7 days they beat UPS). It's exceptionally good if you package stuff properly.
And by properly, you have to anticipate the address label falling off. If this happens, they will open the box to see if maybe there's something with an address inside (a packing list inside the box, and not just in a packet on the outside is a good idea, but only if you include both the sender and recipient addresses on it.
If it's particularly valuable, and composed of a lot of pieces, it wouldn't hurt to individually wrap and address each item. doesn't have to be fancy - inserting each cartridge in a plastic self-sealing bag with an address inside the bag works just fine. In case the box gets totally ripped apart because of machinery or handling and all the parts fall out, each one is individually labelled and can be forwarded on.