Yes you should be able to buy it anytime. Heck they'd probably even sell it to you after the season is over!
You might want to log in to your account and make sure it shows up as an option to purchase. Mine didn't, so I went to a license dealer and they couldn't find it to sell to me either.
I called the WDFW licensing number and after waiting on hold for a while a nice lady got it all straightened out and I went ahead and bought it online. Got it in the mail in less than a week.
Number to call would be 902-2464