What is the burden of proving that the items in the cost bill were not reasonable or necessary?

"If the items appearing in a cost bill appear to be proper charges, the burden is on the party seeking to tax costs to show that they were not reasonable or necessary. On the other hand, if the items are properly objected to, they are put in issue and the burden of proof is on the party claiming them as costs." (Ladas v. California State Auto. Assn. (1993) 19 Cal.App.4th 761, 774 (Ladas).) We review for abuse of discretion whether a costs award is reasonable, but "because the right to costs is governed strictly by statute [citation][,] a court has no discretion to award costs not statutorily authorized. [Citations.]" (Ibid.)