Either "Internet" or "internet" is fine. Webster's lists "Internet" as the primary spelling, but the Cambridge dictionary prefers "internet". Since dannymleone has noted elsewhere that they target British English your correction is probably the better one.

Great catch! I missed that one the first time around.

Thank you, guys, for helping me with my letter.

Thank you so much.

You're welcome! One thing I would note as I look back over this, especially since you said you target British English, is an inconsistency in how you use the comma in lists. You have "Teepublic, Redbubble, Zazzle", with a final comma, but "inspiration, imagination and of course special software" without.

Either format is perfectly acceptable, although using the final comma -- what's often called the "Oxford comma" -- is much more common in American English. British English accepts both, depending on individual or institutional style. Use whichever feels better to you or matches the audience you're writing for, but try to stick with one format.

(You may have used the final comma in the first example simply because you were missing the conjunction but felt like *something* needed to be there. In that case, if we wanted to keep consistency with your second usage, my correction could have been better as "Teepublic, Redbubble and Zazzle".)

As a side comment, it's funny that writing "Teepublic, Redbubble and Zazzle" felt completely *wrong* to me, almost uncomfortable, to the point that I had to actively think about it even this second time I typed it. That's even though I know it's fine, and is sometimes also used in the US even if the Oxford comma is much more common here. Language is weird.

Could you explain to me why you put the word "companies" in plural form?

Sure! Here you are naming Amazon as only one of a group of companies, "the best performing in the world". Since you're talking about membership in a group, the noun needs to be in the plural form to represent the multiple companies in the group.

You would use the singular if you were saying "Amazon is THE best performing company in the world", because there you are only talking about Amazon and no other companies.

Does that make sense?

Oh, I also missed the first time that you have "and the one of the best". That first "the" doesn't need to be there, it should be "one of the best", because again you're talking about one of a group. I know that coming from Slavic languages articles can be tricky. Always try to think about whether you are talking about one particular thing in isolation, or one thing as a member of a larger set.

It's a bit tricky. I'm confused.
I know that Merch by Amazon is a print-on-demand service by Amazon.
So,I assumed, that it some kind of branch of the company. That's why I came up with this question.

Ahh, OK. If you're talking about Merch by Amazon as a branch of Amazon, and saying Amazon is the best performing company in the world, something like "the combination of my experience and part of the best performing company in the world" would be better, because here you are talking about a single company, Amazon. The way I first read it was as though you were talking about Amazon as one of a number of companies in which case the plural is necessary, but if you're wanting to talk about just part of one company the singular is fine.

Either "Internet" or "internet" is fine. Webster's lists "Internet" as the primary spelling, but the Cambridge dictionary prefers "internet". Since dannymleone has noted elsewhere that they target British English your correction is probably the better one.

Great catch! I missed that one the first time around.