Theres nothing wrong in writing that line. 
But I just want you to note that someone else reading it could take it the wrong way as well…They could take it as..“why would you be donating your gifts? Are you too good for what I am bringing specifically for you? My gift was meant for you not for chairty?” “I can drop it off at the local salvation army myself and might as well show up empty handed but then they would just think that is rude.” “Why are these people trying so hard?”
I’m just saying, what you think you are writing is “OK”, because ONLY YOU know your intention behind it which is you do not want people to go out of their way to bring gifts for you because you rather donate them. Thats fine…but that line can be taken way out of context just as the same as “no boxed gifts”.
I think going with “Your duas and blessings are most appreciated.” is better rather than pointing out that whatever people go out of their way to bring for you will be given to donation…because your too good for it or you don’t want peoples gifts (thats up the person reading the card)… And now can I ask, do you plan to donate the cash gifts as well? or Just the boxed gifts?..People can also come to those conclusions that you’ll be keeping only cash envelopes and giving away all the other stuff. So that can look JUST as tacky. That basically suggests you’ll be “giving away” anything you DON’T WANT. It doesn’t really imply the intentions of doing a good deed.
Like, its all one in the same and what makes sense for you, won’t make sense to another…but hey thats fine, because its YOUR day, and you and your family know your good intentions to not care what other people think right?..So neither one of those lines are better than the other…
If the intention is to “give” then you might as give people a charity’s name to directly donate the money and gifts to themselves…rather than leaving people room to make their own absurd assumptions and judgments about you and your family…just like people have in this thread and you have about the people who want to write “no boxed gifts.”