General warning: as soon as you decide to self-publish, a certain section of the population decides that you are a cow to be milked. Some will be blatant about it by sending you emails soliciting your business. Usually, it’s to buy into some form of marketing. Mostly, you’ll be marketed at in more subtle ways, where people tug at you with statements like “I went from selling xxx to selling yyyyy using this method/site!”
Especially the latter is very hard to evaluate objectively, because writers get told to treat their writing as a business, and you should therefore invest in that business, right?
Yes, you should invest in your business, but you should invest smartly in that business. You will not make your book sell better by randomly throwing handfuls of money at it.
In order to know how to invest smartly, you first need to know what you need and who offers the best services to give you these things.
It is perfectly OK NOT to invest terribly much while you’re learning the ropes, especially on the side of marketing.
Don’t become that author with the $2000 book trailer without a clue how and where to use that book trailer (hint: book trailers are a luxury that you can spend on when you can afford it. They don’t lead to many sales).
Don’t become that author with a $1000 book cover by a great artist who 1. has never designed a book cover before and 2. didn’t really portray genre cover conventions that help sell the book.
Don’t become the author who bought a marketing plan from a vanity-type press because the people emailed and it “sounded so good”.
If someone emails you about a service and you have to pay for it, it’s not going to be something you want.
Don’t fall in these traps. Educate yourself. Decide what YOU need and then hunt for people to provide the service. Anyone whose service is good will be very busy and won’t spend much time looking for clients.
No, it’s not easy. Yes, it sounds like work. If it sounds easy and too good to be true, then it usually isn’t. Do your homework. Sit on your wallet until you’re convinced that the service is good. Ask other people about it. Google the service. Ask the Kindleboards hivemind about it. Do. Your. Research.