I want to buy menthol powder from a chemical supplier but he is selling menthol crystal at half price of menthol powder (he says manufacturing process is the difference) so I am really confused what should I do. Can I buy menthol crystal and just powder them to get the powder form I want it or are crystal and powder menthol totally different things altogether?
Unless there is something bizarre going on (which should be mentioned more or less clear on the website/in the catalogue because no chemist would expect it), menthol will be menthol as there is only one compound defined as menthol. This compound is chiral, so I would expect a stereodescriptor. In its absence, I would not be sure whether I am dealing with a racemic sample (more likely in the case of a chemical supplier aimed at practicing chemists) or a natural, chiral sample (more likely in the case of a supplier selling to the general public).
Chiral menthol crystallises easily and nicely, and because crystals are easily separated from contaminants, they are often the preferred form of isolation of compounds for sale; especially since the crystals themselves very rarely contain impurities. While a powder can be the better choice for several purposes (e.g., weighing small sample amounts into a flask), it may not be as simple to produce. In the case of menthol, I expect the powder to derive from powdering the crystals. This is an extra step and thus costs more money.
Depending on how important the quality of the powder is to you, it may make more sense to buy the powder directly or to buy crystals and grind them to powder yourself. However, as I said above, I am very sure that they contain the same chemical in a comparable purity.