1. On Settings > Billing click Buy credits in the SMS Credits box (or go to /admin/directmessages/buycredits). The SMS Wallet page shows your Available Balance, Estimated SMS Remaining, and Last Top-up.
2. Click Add credit to jump to the packs section, then click a pack's own Add $X credit button - packs are $1 Starter, $50 Essentials, $75 Growth, $100 Business, $250 Scale, $350 Pro, $500 Enterprise - or type a custom amount (minimum $50) and click Continue.
3. Confirm in the pop-up ("Are you sure you want to add $X in SMS credits?") with Confirm Purchase.
4. Your card on file (Settings > Billing) is charged the amount plus a 3.5% processing fee; on success you see "SMS Credit Added Successfully" and the balance updates.

Custom amounts below $50 are refused before you even reach the confirmation step, with "Minimum custom amount is $50.00". Each message costs the per-message rate shown on the page (default $0.07).