Yes. If you pay annually, you'll have access to all of your credits right away. For example, if you pay annually for the 50 credits account, you'll be able to use all 600 credits (50 x 12 months) right away. You could send all 600 in one message if you'd like, or within 1 month, 2 months, 3 months, or whatever you choose. If you go over that limit at any time, you'll need to upgrade or pay per additional credit.