When the fall boxes are ready, you will pay on the diygreenlife site. Shipping + $2 charge per strain if I am remembering correctly. YOU get to do the calculations and put in the total paid (i.e. you get 4 strains? $6 + 4x$2 = $14. You can always send more as a site contribution.
During the course of the year, various runs are open, and signups are on each run. Some have a few slots left now, but many are filled up (there are constantly new ones popping up). There was an easy way to see on one page what runs you signed up for, but you can always click the links from @G-paS to search individual runs. For each run you are signed up for, add $2 to the $6 shipping charge to figure out what to pay…