HudumaSMS is prepaid. You buy credits in advance and each message part consumes credits at your account’s rate.

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Rates

Rates fall as volume rises. The tier is decided by the quantity you purchase:
Credits do not expire. Unused credits stay on your account for as long as it is open.

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How a message is counted

One credit buys one message part, not one message. A single accented character, emoji, or certain punctuation switches the whole message to the 70-character limit.
This surprises people writing Swahili with accented characters. A 140-character message in plain GSM text is 1 credit; add one accented character and the same message becomes 3 parts, costing 3 credits.

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Per-envelope charging

Each envelope in a send request carries its own message and is charged separately. Sending to 500 recipients costs at least 500 credits.

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Checking your balance

Every send response includes the balance remaining:
If a request would cost more than your balance, it is rejected with 402 and nothing is sent — it is not partially fulfilled.