From a community member I would have expected some more gratefulness. 
I didn’t know that Fairphone is sending around t-shirts. You probably could have even sold it on the #market because t-shirts are something that others would happily receive: Fairphone T-shirts and more for the community?
Apart from that, the environmental impact of two paper cards is diminishing in comparison with the huge gains we draw from the modular design of the FP2. If you assume recycled paper, an LCA conducted by the Environmental Defense Fund found that the total greenhouse gases* accumulate to 1570 kg CO2e/t paper. A DIN A6 postcard (0.01554 m²) at 300 g/m² thus produces 4.662g * 0.00157kg/g = 0.00732 kg CO2e. According to the FP2 LCA report (PDF (1.6 MB), p. 37) conducted by Frauenhofer IZM, the FP2’s Global Warming Potential (GWP)* is 43.85 kg CO2e. Therefore, if Fairphone sends out 6,000 postcards, they roughly have the same impact on the environment as one FP2.
Now, every time you replace your battery instead of buying a completely new FP2, you save 93.7% of its GWP or about 5,614 postcards. Surely this justifies two thank-you postcards. 
* I haven’t dug deeper into the exact methodology the two reports use to define “total greenhouse gases” and “Global Warming Potential”, but the latter seems to be defined more clearly.