If you already have a mail order operation you will be quite well placed to expand into a wider market. You will have a existing relationship with carriers, and be used to establishing delivery times, charges, charge zones and so on.
Moving to a global market will bring more opportunity, but also more problems, like calculating sales taxes and delivery charges on a world-wide basis. Also, handling returned goods becomes a bit more tricky, as does quoting prices in multiple currencies that can change quite regularly.
Will you have an automatic link to a calculation that revalues each currency in realtime, or simply input that data yourself, and if so, how often will it change, daily? weekly? hourly?
If you plan to be able to deliver on a world-wide basis, you will also need to develop your relationship with your delivery agency, and possibly link your order processing to an electronic order tracking system operated by your carrier. You may also need to get to know your customs offices in a bit more detail.