The common solution to the general problem of how to send mail to a recipient in a foreign country and pay for them to send it back to you is the International Reply Coupon, which is overseen by the Universal Postal Union (of which France and the United States are both members). From Wikipedia:
An international reply coupon (IRC) is a coupon that can be ...
As mkennedy says, it's the floor/flat.
When I lived in Glasgow it was in the top floor flat (4 floors) on the right, so the address was
Flat 4/2, 129 XXXXX Road, Glasgow G12 XXX
There isn't an explicit numbering in the flats, but usual to number from the left (stair) side, so left would be /1, and so on until you include all flats on the floor.
This is absolutely normal. Every country has a maximum amount allowed to be received in a country without the customs duty. The amount you have listed obviously exceeded that amount. And when you mail at least from the US your declared value of the merchandise/gift has to match the amount you're insuring the parcel for.
So since your amount exceeded ...
The post service won't expect a stamp if you are sending something to an “Antwoordnummer”. They can tell and decide who to charge based on the address itself. It doesn't matter if it's a loose reply card, an envelope with the address printed on it or something you prepared yourself.
I think your best bet is a mail scanning and forwarding service. (Google it, there are many, and I haven't used one to recommend.)
Give you a 'personal' address, intended to be professional looking; I.e. A street address with a 'Suite' number or equivalent for your personal account.
Provide a website for viewing all scanned/photoed envelopes/...
The same as if the person has simply moved to a different address in Japan. You ideally put a sticker on, or else in a bold colour, put 'no longer at this address' across the letter, and put it back in the mail. This will usually have it returned to sender in most countries.
Alternatively, go to a post office with the letters, hand them over and ask "...
In my response below, seems I was wrong. USPS M-bags are for everyone (and available at a local USPS office). ISAL M-bags are for commercial use: http://pe.usps.com/text/imm/immc2_039.htm
293 International Surface Air Lift (ISAL) Service
International Surface Air Lift (ISAL) service, including ISAL M-bags,
I cannot read Spanish very well and I have no first-hand experience with this in Spain so I cannot tell you exactly what to do in this case but note that:
There is no allowance for parcels like there is for air travel. If you carry things for your personal use with you by air, even things you just bought abroad, and they are worth less than €430, you don't ...
It is known that the Russian Post Service (Почта России) is notoriously unreliable and slow
Actually, that's no longer true. In November 2016 I've sent two heavy packages using the cheapest regular mail service from Czech Republic to a middle-sized city in Russia. The packages took 3 days to leave the Czech Republic and then took only 10 days to get ...
In amateur radio we are often wanting to cover a foreign station's postal costs to send us a contact card (QSL card). The usual solution is to send a couple or three American dollar bills, as most people in most countries can convert this to local currency fairly easily.
The UPS Store seems to be offering everything you are asking for. From their website:
Here’s what you get with mailbox rental:
A street address, not a P.O. Box number
Package acceptance from all shipping carriers
Mail holding and forwarding
24-hour access (additional fees apply)
(And some other stuff I removed because it's marketing.)