Just a reminder as a QSL manager for many stations, some QSL managers cannot accept stamps as return postage. For me QSL = $2.
Without the $ I would not be able to provide a free Bureau service. In 2014 i sent out 31,000 bureau cards which taking into account of printing and carriage costs it works out as £1,600 and this would not be possible if stamps were used.
Another consideration is that from the United Kingdom although some letters can weigh 10g 97p, most will weigh 12g or more which is £1.28 (over $2)
Many UK QSL managers use a mailing contract using Postage Paid Imprints,(not stamps) and on this contract letters to USA can be as little as 60p. So for us using stamps is just throwing money away which is better invested in our bureau costs and other DXpedition costs.
Some may say that making a business out of QSLing is just not on! But we have a duty to provide the best quality of service for both you the DX chasers AND the DXpedition teams. Regrettably I have received a lot of envelopes with insufficient stamps either I have to use $ to put the extra stamps on or the QSL goes Via Bureau.
Hope this explanation helps a little? Have a great 2015 to you all, have a safe New Year!