Meghan Markle has joined the British royal family more than a year ago. She could be one of the most frugal members of the royal family but, still, she allegedly causes financial headache to them. 

Prince Harry and Meghan Markle just concluded their first royal tour with their son, Archie Harrison Mountbatten-Windsor. The family spent 10 days in South Africa. 

But, the expenses of the Sussex couple's travel across the world like this is not the main financial burden for the monarchy. It is Meghan's citizenship. 

Though she is married to a prince, who is a British citizen, Meghan remains an American citizen. She has applied for British citizenship, but it takes time for the application to be approved. 

This means the former Suit actress is still an American citizen, which makes all her earnings subject for U.S. taxes. She is required to pay taxes in her birth country for any income earned or allowances she receives. The taxable income includes items such as homes, gifts, and earnings from her acting career and endorsement. 

Moreover, since Harry is married to Meghan, the U.S. could also attempt to request taxation from his income as well. This could mean all of his inheritance from his mother, Princess Diana, and her great grandmother, the Queen mother, could also be subject to U.S. taxation. 

Prince Harry is already paying annual income tax in the U.K. for the £10 million that Diana left for him and the £7 million inheritance from the Queen Mother. Additionally, any income that Meghan and Harry's son, Archie, would earn could also be taxed by the U.S. as well. 

Daily Express UK cited the U.S. Internal Revenue Service's law that states its citizens are obliged to file a U.S. tax return and to report any foreign accounts, assets more than $200,000 (£162,458), and gifts valued more than $15,797 (£12,831) regardless of whether it is actually in their possession.

Apparently, there is only one way for Meghan to avoid paying U.S. taxes, which is to renounce her U.S. citizenship once she becomes a British citizen. However, even if she does so, Archie still needs to pay U.S. taxes until the age of 18.

In related news, Meghan has filed a lawsuit against British newspaper, Mail On Sunday, over the publication of a private letter. In his statement, Prince Harry compared the situation that his wife is in right now to that of her late mother, Diana. The Duke of Sussex expressed fear that Meghan will suffer the same fate as her mother.

"My deepest fear is history repeating itself. I lost my mother and now I watch my wife falling victim to the same powerful forces," he said. 

Harry did not mention a specific letter on his statement. However, the letter is believed to be the one that Meghan sent to her estranged father, Thomas Markle, which the tabloid published earlier this year.