Implement `sdk/src/utils/feeCalculator.ts` for pre-transaction fee estimation

[Levi-Ojukwu]

Description
Integrators need to estimate Soroban transaction fees before signing to give users accurate cost previews. The SDK should provide a fee estimation utility that simulates transactions and returns the fee in XLM.

Requirements and context

• estimateFee(operation: SorobanOperation): Promise<{ feeXLM: string, feeStroops: bigint }
• Internally uses Soroban's simulation endpoint to get the resource footprint
• Add a feeBuffer parameter (default 1.2x) to the estimate to avoid under-budget failures
• Expose per-method convenience functions: estimateSubmitFee(params), estimateFundFee(invoiceId)
• Write tests with mocked simulation responses

Suggested execution

git checkout -b feat/sdk-fee-estimator

• Create sdk/src/utils/feeCalculator.ts
• Write unit tests for each convenience function
• Document in sdk/README.md

Example commit message
feat(sdk): add fee estimation utilities for pre-transaction cost preview

---

[YfengJ]

Hi maintainers, I would like to work on this issue if it is still available.

I would keep the implementation limited to SDK fee estimation only: add a feeCalculator utility around mocked/testable Soroban simulation responses, expose the requested convenience helpers, apply a configurable fee buffer, and document usage in the SDK README. I would not touch signing, key handling, wallet flows, or real transaction submission behavior.

I will wait for Stellar Wave or maintainer assignment before starting implementation.

[Stephan-Thomas]

@Stephan-Thomas has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hi, i would like to work on this.

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[madisonsc52-del]

@madisonsc52-del has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, I would love to work on this issue. I can get started on it right away. Could you please assign it to me

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[DTriple-p]

@Agbasimere has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello, I'll like to work on this issue.

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[dotmantissa]

@dotmantissa has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Please, assign this issue to me.

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[bbjiggy]

@bbjiggy has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, I'd like to work on this issue. Please kindly assign this to me

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[Malcolm-Wander]

@Malcolm-Wander has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer,I’d love to work on this issue please…promise to do an accurate and swift job,thank you.

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[Rendezvous-droid]

@Rendezvous-droid has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

can i work on this, please?

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[Ultra-Tech-code]

@Ultra-Tech-code has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

hello, I'd like to work on this issue.

I'll implement sdk/src/utils/feeCalculator.ts to provide Soroban transaction fee estimation utilities. I'll add estimateFee() using Soroban's simulation endpoint to calculate resource requirements, include a configurable fee buffer (defaulting to 1.2x), and add convenience helpers like estimateSubmitFee() and estimateFundFee(). I'll also add unit tests with mocked simulation responses and document the new SDK fee estimation functionality.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Ultra-Tech-code to this issue.

[Caneryy]

@Caneryy has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello, I'd like to work on this issue.

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[drips-wave]

Congratulations, @bbjiggy! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @bbjiggy: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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[grantfox-oss]

🦊 GrantFox — @bbjiggy has been assigned to this issue!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #373)
2. Your PR will be reviewed by the Invoice-Liquidity-Network maintainers

Good luck! Track your progress on GrantFox.