Guyana do not make use of zip code or postal code. Zip code is used in some Countries like Canada, USA etc but it is not used in Guyana. That shouldn’t stop you from using a zip code,  Most people have resorted to using 00000 or 12345 as Guyana zip code.  keep in mind that 00000 or 12345 is not the zip code for Guyana but you can use it since Guyana doesn’t make use of zip code. read the article below to fully learn what zip code is and how it works.
Is there any difference between the Guyana zip codes, Guyana Postal Code
From the definition above you would agree with me that they all means the same thing only that they are called zip codes in some countries like USA and then postal codes or post codes in other countries. To know more about the uses and the importance of a zip code/postal codes read this article>> ALL YOU NEED TO KNOW ABOUT ZIP CODES AND POSTAL CODES
What will happen if i use a wrong  Postal Code or zip code while shopping online?
When you use an invalid zip code/ postal code online since Guyana has no zip code. nothing will happen and your order will still be delivered to you, using your address information you provided while filling the form.