Mont. Code § 15-31-122
Pursuant to the provisions of Article III, section 2, of the Multistate Tax Compact, every corporation deriving income from sources both within and without the state of Montana and required to file a return and whose only activity in Montana consists of making sales and which does not own or rent real estate or tangible personal property within Montana and whose annual gross volume of sales made in Montana during the taxable year does not exceed $100,000 may elect to pay a tax of 1/2 of 1% of gross sales made in Montana during the taxable year. Such tax shall be in lieu of the tax otherwise imposed under 15-31-101 and 15-31-121. The gross volume of sales made in Montana during the taxable year shall be determined according to the provisions of Article IV, sections 16 and 17, of the Multistate Tax Compact.
§ 15-31-122, MCA