I forgot to mention that there's a way to get 19 envelopes:
STEP 1. Go to the online app and click on the "More" tab, which is located in the lower right corner.
STEP 2. Select "Invite your friends".
STEP 3. Copy the link that appears.
STEP 4. Open a tab in incognito, paste the link, click on "Start playing" and wait for the interface to finish loading.
STEP 5. Close the page and repeat the process. No need to register with new accounts.
At 10 invitations they give you four envelopes, At 20 invitations you receive 5 more, and 10 more when you repeat the process 40 times.