Global Mail has the possibility to also get assessed customs fees. However, because it uses postal clearance instead of express clearance like DHL International Shipping it is less likely to get assessed duties and taxes as well as additional charges.
As well as customs Duty, customs can apply a VAT (Value added Tax). This basically means that customs can change the value of what you ordered to what they think it is worth and not what you paid for it. Customs may also tax you on the cost of shipping and include that in the value calculations. There are a lot of ways in which customs can increase the tax be significantly higher than what you paid for the items.