The voucher can only be used to purchase products that AndyMark sells and is unrelated to how your team can get items from FIRST Choice.
FIRST Choice is managed by AndyMark, but it's part of your Kit of Parts you get with your FRC registration. Each team will be issues points and each team can make a wish list of items. This year's FIRST Choice system hasn't be release (afaik) but you can review
last year's process and get a good idea.
If you're team isn't completely sure they are ready to design a custom drivebase, it's probably best to not opt-out.