A very polite way to explain why you're doing something is "I wanted to ___". For example:

Hi Jen. I wanted to see if you'd like to come out with Emma and I and some friends of ours to a show next week.

Yes, I just wanted to verify that my payment has been received?

Some phrases that can be used after "wanted" include:

I wanted to ask...

I wanted to tell you...

I wanted to check to see if...

I wanted to remind you that...

I wanted to let you know that...

I wanted to find out whether...

Note that this is in past tense: you say "I wanted to ___" instead of "I want to ___". There's no grammatical reason for making it past tense; it just makes your sentence sound more polite. When you say it this way, it sounds like an explanation for why you're calling, why you're visiting, etc.