A credit card is open ended which is 3 years. No, signing a letter does not make it a written agreement as far as SOL is concerned, however, it may re-set the SOL. I spoke with a consumer attorney here in WA and was told both parties need to sign the agreement. This same attorney also told me it is not illegal for a CA not to validate. I would suggest contacting an attorney in the local area for a free consult.