Where an on-chain fee is charged from

How on-chain fees are charged for transfers from Cobo Portal and for API transactions, and the order Billing Center charges gas tokens and Credits.

An on-chain fee can come from three places: gas tokens in the wallet address, gas tokens in Billing Center, and Credits in Billing Center. Which of the three is charged depends on your configuration; Portal transfers and API transactions are configured differently.

Transfers initiated from Cobo Portal

The transfer dialog lets you choose what pays the fee — “Use Billing Center” or “Use the token in the wallet address only”:

Chain	Use Billing Center	Use the token in the wallet address only
EVM-compatible chains	Uses the address’s gas tokens when they are enough; otherwise Billing Center pays	Uses the address’s gas tokens only, and fails if they are insufficient
TRON, Solana, Tempo	Billing Center pays directly; the address’s gas tokens are never touched	Uses the address’s balance only, and fails if it is insufficient

The choice is made in each transfer’s dialog and applies only to that transfer; Billing Center has no global switch governing Portal transfers.

Transactions initiated through the API

How the fee is paid is determined by the auto_fuel parameter in the request:

Value	Behavior
ProActiveAutoFuel	Billing Center always pays, without touching the address’s gas tokens
PassiveAutoFuel	Uses the address’s gas tokens when they are enough; otherwise Billing Center pays. Not supported on TRON, Solana, or Tempo
UsePortalPreference	Follows the per-chain sponsorship rules configured in Settings
DisableAutoFuel	Never uses Billing Center; the address pays and the transaction fails if the balance is insufficient

When the parameter is omitted it defaults to UsePortalPreference, which follows the Settings configuration. When another value is passed explicitly, the request decides and the page configuration is not read.

The two settings under Advanced settings

In Billing Center, click the Settings tab and expand Advanced settings. Both of the following apply only when an API transaction passes UsePortalPreference, or omits the parameter:

• Per-chain sponsorship rules: set sponsorship per chain. EVM-compatible chains offer three options — use the wallet address only, sponsor when the address balance is short, or always sponsored by Billing Center. TRON, Solana, and Tempo offer only two: use the wallet address only, or sponsored by Billing Center.
• Which balance to charge when sponsoring: either “Gas tokens first, Credits when short” or “Gas tokens only”; with the latter, an API transaction on that chain fails when gas tokens fall short.

One fee is paid in full by a single source

When a source is not enough, the whole fee moves to the next source rather than being topped up.

For example: the address holds 0.003 ETH and the transaction needs 0.005 ETH. Billing Center does not add the missing 0.002. It pays the full 0.005, and the 0.003 already in the address is left untouched.

The same holds inside Billing Center: that chain’s gas tokens are charged first, and when they fall short Credits pay instead — the gas token balance is not spent first and topped up with Credits.

This matters for reconciliation: every fee in your transaction records is tagged with exactly one payment source — Paid with Credits, or Paid with gas token.

Common questions

The wallet address holds SOL. Why was it not used?

On Solana and Tempo, as soon as a transfer goes through Billing Center, Billing Center pays the fee in full and the address’s balance is never touched. This is not a fallback triggered by an insufficient balance — it is how these two chains always behave.

To spend the address’s own balance, choose “Use the token in the wallet address only” in the transfer dialog. Note that this is all or nothing: nothing is sponsored, and the transaction fails outright if the address’s balance is insufficient.

Can I charge fees to Credits only and leave gas tokens alone?

There is no “Credits only” option. As long as Billing Center’s gas token balance for that chain covers the fee, gas tokens are charged first.

If you want a chain’s fees to come only from Credits, simply do not deposit gas tokens for that chain; its fees are then always paid with Credits.

I changed a per-chain sponsorship rule in Settings. Why did Portal transfers not change?

That setting applies only to transactions initiated through the API. Portal transfers behave the same way per chain every time, and can only be changed individually in each transfer’s dialog.

Why does each fee in my transaction records show only one payment source?

Because one fee is paid in full by a single source — two sources never each cover part of it.