I feel that questions regarding the working of various postal networks and services etc. should be on-topic for this site as Philately comprises this study as well. Wikipedia's Diversification section of Philately supports my argument- and in the end, our goal should be to diversify (pun intended) the proposed site.
In that case, the proposal should be modified to Numismatics & Philately - Coin & Postal Study or so. The description should be changed accordingly.