How do you account for travel expenses incurred to purchase raw materials?

Travel expenses incurred for the purchase of raw materials can be allocated to "manufacturing overhead". But if the travel costs are counted in the "manufacturing costs" to go, it means that this part of the cost of allocation (because manufacturing costs at the end of the month to be allocated between different products, finished goods and products), the practice is more complex, and most companies have no reasonable allocation of the basis. Therefore, in practice, due to the purchase of materials incurred by the travel expenses, generally is categorized as "administrative expenses - travel expenses". I think the practice of charging to "administrative expenses" is more simple and feasible.