I can't see anywhere a comma would fit in either of your sentences as they are currently. The SE question you linked is about using commas after introductory clauses, which neither of your questions have.
An example of a sentence with an introductory clause would be:
In Palm Springs, I can be sure the rain wont interfere with my golf practice.
The introductory clause here is in Palm Springs and the main clause is I can be sure the rain wont interfere with my gold practice.
However, these commas are entirely optional and if you can avoid them you should. Personally, in the above sentence I wouldn't use a comma.
Also, there are a few errors in your first sentence, and fixing one of them results in an introductory clause:
In June, you are virtually guaranteed to get a good tan in the Caribbean or the Mediterranean region.
Firstly, adverb order makes the in June in the Caribbean sound awkward. Normally, we'd just rearrange the adverbs but in this case you need them in this particular order for the sentence to make sense. So, we make the in June part an introductory clause. I put the comma in because it further removes in June from the other adverbs (making the fact that we are ignoring adverb order less obvious).
Secondly, you can virtually guarantee to is a common mistake. The grammatical version is you are virtually guaranteed to.
Finally, the Mediterranean can't be used like the Caribbean. The Caribbean is a name on its own, but the Mediterranean isn't (unless you're talking about a hotel called 'The Mediterranean' or something). Instead I used the Mediterranean region, but area or some other noun works fine.
