слова "till" нет. есть слово "until" лучше сказать "I hereby confirm" или "This letter is to confirm that..."вместо "I will take care of expenses" сказала бы "I will cover all expenses of...."*ред.* что-то я сложнила  в итоге я бы написала так:This letter is to confirm that I, _____, will cover all expenses of _____, _____, and _____ during their trip to _____ from _____ through _____.