I found this thread really thought-provoking and wanted to join in. I am a Christian and I love to look at things objectively.
I think it's perfectly reasonable to choose not to donate to causes that represent or proclaim beliefs that you disagree with. Samaritans Purse (and Operation Christmas Child) should be clear(er) about who and what they are.
May I offer another perspective on how charity and mission can coexist, without charity just being the gift wrap of an 'ulterior motive'?...
Christians believe that there is a God who loves people and because they have experienced this love themselves, they want to show love to others. This could and should be able to stand without any mission attached. However, pretend for just a sec that you are convinced, beyond all doubt, that Jesus is the way for ever-lasting life. How much would you have to hate someone, not to tell them? I mean, even if I've seen a good offer in Tesco I want to tell people! I think that is why love and mission can go together in an ideal world. In the non-ideal world, Christians get it right, get it wrong, screw it up, try again and manage something somewhere in the middle.
A faith based mission organisation should be sensitive and respectful to local culture and existing beliefs, offering information but not becoming too assertive. Certainly they should make sure their gifts are truly gifts and have no obligation attached. When a faith based group proclaims a message though, naturally this message is going to contradict other faiths.
It's unpopular, seems old-fashioned and it's a sore subject to suggest, but Christians believe that logically, there must be one truth that is true. So, from a Christian perspective, it's not a case of competing with other faiths, like Pizza Hut competes with Pizza Express and Dominoes for faithful customers but it's all pizza at the end of the day. Inherently then, Samaritans Purse would not say that any other faith is correct. But it should love and respect people with other beliefs.
I'm not comfortable with the booklet's certificate at the end, because I think it's pointless. If a child (or adult) chooses to pray or to have a faith, this is likely to be a process of thought and consideration that they will continue to have into adulthood. Writing down a date and time isn't signing up to anything. Christian faith is all about what's going on in the heart and mind, not what's written on paper or whether someone attends a church building.
There certainly should be no implication that Christians are nicer, smarter, 'better than you' etc. The absolute core of Christianity is that Christians are messy, imperpect, ordinary people who screw up and have found a source of grace.
So I think it's not unreasonable to opt out of Operation Christmas Child or to choose a different charity if you want to give a shoebox without giving out the Christian message too.