The website limits membership to the state of WA and certain ID counties. However, the online application includes the option of association membership via the Inland Empire Philatelic Society. You join the Society separately by printing and completing its membership application (available on the IEPS website) and remitting it along with modest dues. STCU waives the $20 membership fee if joining through this association.
The 5-year certificate (2.38%APY) continues to be available, and compares favorably to what could be obtained elsewhere (example 2.75% APY for 5 years at Salem 5 Bank), especially for an add-on. Additional deposits are unlimited, however there are constraints on how often and how much can be added at one time as mentioned in the original post. https://www.stcu.org/rates/Certificates.aspx
Early withdrawal penalty is 180 days of dividends.
When joining online, note that the 48-60 month certificate included in the online application is NOT the one offering the 2.38% APY
The online application offers an option to fund through a credit card, so it is possible to fund the savings account with the combined minimum deposit for the savings account and any other accounts you wish to open. The credit card charge for the opening transaction was flagged as a mail/phone order. Some credit cards flag transactions from financial institutions as cash advances so YMMV. The credit card funding option is available only for the initial account open.