As a sender of a proposal, you do not need to countersign a proposal. Only one signature box can be inserted per proposal, and there is good reason for this.
A little history lesson
The original purpose of countersigning was introduced for paper documents to certify that no party had changed either version of the document.
Along with the evolution of digital documents, this requirement is negated and so only one person's approval is necessary - that of your client.
For more on the legal aspect of digitally signed documents, check out our article here.