There is nothing critical in the mailing. Everything you need is on the portal. The main mailing contains prepaid envelopes, but you can accomplish everything without them and then wait for them to use for mailing or just pay your own postage with your own envelopes. Important thing is to get the items completed; don't wait for the envelopes to arrive.
The other mailing is from the security office and only contains fingerprint cards and the same instructions that are on the portal. As long as your police department has authorized fingerprint cards (SF87 or FD258), you can use those. Even easier is to go call a local Army recruiting center and get them done electronically using the codes provided in the security letter on the portal.