I agree with the sentiments of bubbleOseven regarding an act of altruism, and that a Thank You would be nice, but no, is not something I would have expected.
It is always nice to be appreciated, but as BS has indicated, some charities regard any donations as a green light to ask for more and more. Under the influences of them now taking on commercial marketing people, the thought of a thank you from a big charity is now something (sadly) one might expect to be the exception not the rule.
It's a sad fact of life that when members of society no longer consider a 'please' or 'thank you' (including our elders, dubiously called our 'betters'), then the level of expectation concerning 'good manners' has to be toned down several notches so as not to be disappointed by the majority, these days.
I know it would be nice to get some appreciation, OP, and applaud RP's husband for his efforts, but if someone has no willingness to offer even a word of thanks for some kindness in passing, the sending of a letter or card is likely also to have dropped off their radar.
As a pedestrian, I go out of my way to invite car drivers to make use of a gap (when I know my speed to cross that gap is limited, and they are holding up a queue of traffic by waiting for me). A minority acknowledge me.
The bulk are selfish b'stards who think of number one all the time, and perhaps did not even consider that I might 'dare' cross in front of their car when they wish to enter the side street. For some drivers, pedestrians are a nuisance and routinely ignored (esp in supermarket car parks where indicators seem to "stop working", or is it the drivers' brains ?)