If you write the recipient’s address front and centre, and mark your address ‘sender’ and write it small in a corner somewhere there should be no confusion. Remember to leave space for the customs label though!
There is a new international ‘large letter’ rate that you might qualify for with a jiffy bag if you pack carefully.
Out of the hundreds of parcels I’ve sent out over the years I can count the number that have gone missing on one hand so I wouldn’t worry unnecessarily. I only use trackable shipping for large value custom orders. Do be aware though that parcels generally take around 2 weeks or so and not the 5-7 days quoted on the RM website.
I’m just going to say again that you should make sure you tick the box marked ‘OTHER’ at the top of the customs label - the other options all relate to duty-free goods which yours (being something you have sold) are not. Not that there will be any duty to pay unless it’s a super-expensive thingummy!