Church donations and tax law - how to determine what is/is not deductible?

If you pay the cost of a service the church provides such as a week at camp, it is not deductible because you are buying something they are selling. It is the same as what you would pay at a for profit camp (except it's cheaper.) The costs of the mission trip are donated to cover the costs of providing a service to others, namely the people who will be blessed by your missionary efforts.