If a supplier or vendor was cooperative during a difficult time, it's important to acknowledge it, especially if it may have generated some hard feelings.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I want to thank you for your cooperation during the last <time period>. This was a difficult <time/project/period> for us and it was great having a <supplier/vendor/partner> who was willing to ride out the storm with us.
Your professionalism and patience have paid off and we now <what was accomplished>.
We really appreciate your support during this difficult time. Thanks.
encl: <List of enclosed items goes here>