I'm famaliar with this issue, and I see several approaches.
First of all, French postage is not valid for sending mail FROM the US to France.
For this response, I'll assume that your foreign contact who refuses to use their own postage meter is a US authority; But the following advise works equally for any other foreign contact that is unable or unwilling to use their own postage meter.
Option 1: Use a private carrier
If money doesn't matter, you can use DHL, Fedex or UPS for having your papers sent back to you. You simply buy the shipping label online and enclose it with the papers you're sending to the US authority in question. Some agencies will require that you also supply the return envelope. Note that the US Department of State doesn't like Fedex.
Downside of this option is that it's expensive. You have to throw about €70 one-way for a lousy letter.
Option 2: Buy USPS postage on ebay
USPS postage is sold online; However, USPS refuses to ship physical postage stamps abroad and it's impossible (or at least, very difficult) to buy USPS online postage from outside the country.
So you need a way to work around that, and the easiest way to do that is Ebay.
A few years ago, I tried to buy USPS postage (mint) on ebay, and for some reason, I was unable to find any. All I could find was used stamps for collectors. This, however, seems to have changed. You can now buy USPS postage stamps from several random sellers on ebay and have them shipped internationally.
Option 3: Use an international business reply envelope
Another option I know of is the international business reply service. If you want to go this route, you need to enclose an IBRS envelope with your request and maybe add a note that the envelope is to be used with USPS. This special envelope will then cause the postage to be billed in reverse - I. e. instead of having the sender pay for it, the recipient (you) has to pay for postage.
There are, however, some constraints to be aware of:
One-way countries
First of all, the recipient address on the IBRS envelope (i. e. the issuer of the envelope, you!) must be in a country where IBRS works in both directions, inward and outward. Afaik, France is a country with two-way IBRS; But there a few countries where IBRS works only for outward mailing, preventing businesses in those countries from issuing IBRS envelopes.
Licensing requirements
Some countries require some form of license, registration, monthly fee, minimum volume et cetera. I don't know how La Poste goes about this. What I do know is that Deutsche Post AG has the most convenient IBRS licensing system: In Germany, you can simply print your own IBRS envelopes and hand them to people in other countries. All you need is a German address, and postage will be collected cash by the mailman. This makes IBRS usable and convenient even for one-off arrangements. France however, may be more sophisticated or more expensive.
Size and weight limits
This is another important point: In most cases, the weight of an IBRS item cannot exceed 50g. Higher limits may be agreed on between postal operators on a bilateral basis. This means that if the document to be returned weighs more than 40g (+5g envelope + 5g payment receipt), you can't use IBRS!
No registered mail
IBRS service cannot be combined with registered mail unless the sender chooses to override IBRS entirely by paying for the registration fee AND all of the incurred postage.
Conclusion
If I have to supply a SASE (self-addressed and stamped envelope) to a US agency, it will probably either be IBRS or I'll buy USPS postage stamps on ebay.
Also, keep in mind that you don't have to use the same carrier for both directions. If you're into doing a compromise, you can, for instance, use La Poste for sending to the US and have the return sent with Fedex.
Discouraged Options: Cash, Reply Coupons
I would recommend against sending Cash or Reply Coupons because I don't think that US authorities will bother to exchange those for postage stamps.