Is it legal to use our courier service to buy a lottery ticket ?

Yes, it is legal. It is the same as if you had bought your tickets during a holiday in the country where the lottery is held. None of the lotteries requires that the ticket holder be a citizen or resident of their country. So, if you have the ticket, you are entitled to your win.
If you wanted to, you would also have the option to collect your winning ticket at a location of our choice (in the country of the lottery). You could then claim your prize in person, in the country where the lottery took place. This is an option, not a requirement. Most players are happy to rely on our service to handle payouts and receive their winnings as a check or bank transfer. This is a service that we offer free of charge.