Improve coverage on transaction fees

[filipefurtad0]

What we should change and why (this is tech debt)

Two regressions were observed (and fixed) recently:

• backoffice: failed payments attempt add multiple transaction fees - this is related to issue Unsuccessful SCA payments repeatedly introduce transaction fees (backoffice) #6683
• shopfront: transaction fees removed from payments placed through the shopfront - this is related to a regression not signaled by our build, see this comment.

While these cases were observed in the context of Stripe-SCA, these regressions could potentially affect other payment methods/transactions.

Context

See above and this comment.

Impact and timeline

Relevant to improve confidence when adding new features / upgrades affecting payment gateways and transaction fees.