No, you can only use British stamps for mail posted in London. (You wouldn't expect to be able to use a stamp from a different country on a letter mailed in the USA.)
The cost of mailing a letter to Costa Rica will be different, depending on what country you are in when you mail the letter.
Postal delivery varies from country to country, state to state. Also there are many ways to mail a letter for instance there is airmail which is faster then there is standard mail it's slower or there's priority mail, international for other countries and they all have a different time frame.
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Yes, you can send legal drugs to a different country by mail depending on the rules and regulations. There are companies that are licensed to do this.
It depends where you are posting from. Rates differ from country to country.
A certified letter is a type of mail that requires the recipient to sign for it upon delivery, providing proof that it was received. This is different from regular mail, which does not require a signature and may not provide confirmation of delivery.
that all depends from what country you are sending if t from
Providers for online mail management can vary from country to country. There are different products available but one needs to take into consideration the cost and effectiveness of the mail management.
It depends from where you are sending letter. From the US, http://ircalc.usps.com/?country=12313
no, but you need a stamp on the letter, you can buy stamps at grocery stores in some states and mail the letter in a secure mailbox , not necessarily at the post office.
Clarification needed. In what state or country?