Letter of undertaking

Hi,

I am responding to a tender for supply of goods, some of which will be supplied but third party suppliers. The tender asks me to get a letter of undertaking from these suppliers. I feel like I should write it and send it to them and ask them to put it on to their letterhead, sign it and send it back. This way I'm saving them doing the work and also my submission to the tender will have the same letter from all suppliers. The question is, what should I write?

Thanks