DepositFix lets you set a fixed tax rate. You can also choose to calculate the tax rate automatically. We use a service called Tax Jar which calculates the tax based off of your location and the location of the customer. Before being able to use the automatic tax calculation feature, you'll need to set your company address under Settings.
Then, taxes will only be charged if you and your customer are from the same country. For example, if you and your customer are both from the US, tax will be applied at checkout. But if the customer is from Europe, the tax won't get charged.