Remember, variable costs are NOT defined as those that change over time.
Variable costs are costs that change when one unit of good/service produced changes.
SG&A is the building and maintenance of assets. Assets are fixed costs.
However, maintenance for anything that needs it for each addition use is variable (oiling, repair worker hours, etc. etc.)
Same logic applies to transportation costs! Transport of oil through pipes is fixed (build the pipeline and then just pump oil through). Transport of oil through tankers/trucks would be variable (you need x more for x additional amount of oil produced)