Credits are the platform’s unit of charge. Every outbound message is charged against them: campaigns, single sends, replies from your inbox, auto-replies to inbound keywords and calendar reminders.

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You are billed per part, not per message

A message is split into parts before sending, and each part is charged separately. So a 200-character Latin message is two parts and costs twice a 150-character one. See Message encoding for why the alphabet changes the limit.
For a large campaign, trimming the message back under one part is the single most effective cost reduction available to you. On 10,000 recipients it halves the spend.

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Credits are decimal amounts, not message counts

A balance of 15 is not fifteen messages. Credits are a decimal currency-like amount, and each part costs a fraction of one. Your transaction history records the exact cost per part that was applied, so that is where to look rather than inferring a rate.

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When credits leave your balance

This order matters, because it explains a balance that moves and then moves back. If your balance is too low, nothing is sent and nothing is charged. The request fails and the send button is replaced by Add credits.

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Reading your transaction history

Every movement is recorded on Billing and usage.

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Transaction types you will see

Credits returned after a failed send are recorded as CREDIT, not as a refund. There is no separate refund type in your history.Two consequences: the total added figure in your credits summary includes returns, so it does not represent credits purchased; and to find returns you have to read the descriptions rather than filter by type. Your balance is always correct.

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Getting more credits

Credits can be purchased by card, from three places: the credit indicator in the header, Settings → Billing & Usage, and the Add credits button that replaces Send when your balance is too low for the message you are composing. Choose a package or enter a custom credit amount, pay by card, and the balance updates immediately. Full instructions are in Billing and usage. An administrator can also add credits directly, which is how accounts funded through a Primetel account manager are topped up.
If the credit indicator is missing from the header entirely, the balance could not be retrieved. That is not the same as a zero balance. Reload the page before assuming you are out of credits.

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Controlling spend

Your credit balance is what governs how much can be sent. To cap spending over a period, hold only the credits you intend to use in that period.
Enable Low balance alerts on Profile so you are notified before the balance runs out, rather than discovering it when a campaign stops part-way through.

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Things that cost credits and might not look like they should

A reply is an ordinary outbound message at normal cost. Inbound messages are free to receive; replying is not.
Charged to you, and sent for every matching inbound message. A keyword on a busy shared number with an auto-reply configured will spend credits continuously without further action from you.
Each reminder is a normal message. A rule covering a busy calendar spends accordingly.
The Test button sends a real message and charges for it.
Billed to the account that owns the sending email address, at normal rates.

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New accounts start at zero

An account cannot send until an administrator adds credits. You will already have a usable sender ID, so credits are typically the only thing standing between a new account and its first message.