Ethics - best practice - a requirement or recommendation?

A bit confused with the use of the word “best practice”

The standard on Independence and Objectivity states “best practice dictates that members and candidates always use commerical transportation at their expense or at the expense of their firm rather than accept paid travel arrangements from an outside company. Should commerical transportation be unavailable, members and candidates may accept modestly arranged travel to participate in appropriate information-gathering events, such as a property tour.”

Is accepting paid arrangements from an outside firm, when commerical transportation is available , but the cost amounts to under $100 (i.e. modestly priced) a violation of this standard?

As always, much appreciation in advance!