fix: magic strings and mutation are bad

[0xApotheosis]

Extract the non-fee parts from https://github.com/shapeshift/lib/pull/1187

[shapeshift-ci-bot]

:tada: This PR is included in version @shapeshiftoss/swapper-v17.0.2 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: