All monetary contributions from political committees, political parties and caucus political committees must be by written instrument (e.g., check, money order, cashiers check, credit card transaction, wire transfer).  Those from individuals, associations, unions or businesses must be by written instrument if the contribution is more than $100.  RCW 42.17A.475

Bitcoin and other cryptocurrency should be treated as the equivalent of cash and limited to $100. The cryptocurrency should be converted to legal tender, deposited in the campaign depository within five business days of receipt (RCW 42.17A.220), and timely reported. If cryptocurrency is paid directly to a vendor for goods or services, it should be disclosed as an in-kind contribution using the fair market value of the cryptocurrency at the time the payment is made. The in-kind contribution must be timely and accurately reported by the recipient, including the name and address of the vendor.

Cash contributions must be deposited into the campaign bank account, and not spent directly or mingled with petty cash.  Monetary contributions are required to be deposited into the campaign account within five business days of receipt. RCW 42.17A.475.