A lot of companies want you to review their products for the price of the product itself. Some bloggers are OK with that, others are not. It is up to the blogger, but I agree that you shouldn't have to work for free if you choose not to.
I would suggest making a "work with me" page and outline your conditions - such as a sponsorship/writing fee. If companies are not OK with that, they will be made aware and will not proceed. You might also add that you will not send the product back should you end up not liking it and your review would not be favorable - it is up to them if they do not wish an unfavorable review to be posted - but that they should state that up front. Otherwise, you will post your honest review and move on. I also include in my terms that any giveaways will be provided by the contact so I don't have to incur shipping costs, or have to go to the post office on my own time.