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Bounds are the most under-read part of a rate card, and they are the part most likely to change which option is cheapest for you.
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Key takeaways
Above the cap, the fee stops growing. A rail charging 1% capped at 20 units costs 1% up to 2,000 units and a flat 20 units thereafter, so its effective rate falls continuously: 1% at 2,000, 0.5% at 4,000, 0.2% at 10,000.
This is why capped rails dominate large-payment comparisons and look unremarkable on small ones. If your payments are large, the cap is the most important number on the page.
A minimum fee sets a hard ceiling on how cheap a small payment can be. A rail charging 0.5% with a 1 unit minimum costs 10% on a 10 unit payment. Micro-payments are where floors do their damage.
A cap may apply to the percentage component only, or to the whole fee including the fixed part. Those are different prices, and rate cards are not always explicit. When it matters, ask which one the cap bounds.
It sets the most a payment can be charged. Above the amount where the cap kicks in, the effective rate keeps falling as the amount grows.
It sets the least a payment can be charged, which makes small payments expensive. A rail charging 0.5% with a minimum of 1 unit costs 10% on a 10 unit payment.
Not always. A cap may bound only the percentage part or the whole fee including the fixed part. Rate cards are not always explicit, so ask which one it bounds.
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Prices change. Everything here is explained against live fee data, so you can check any of it directly.