If you send the form and you make a sale in the USA and your book has a free Lulu ISBN, then the tax withheld will be reduced to the rate agreed between the USA and Finland if there is a tax treaty between the USA and Finland.
If you decide to send the form, you should mail it to:
Lulu Enterprises, Inc
Attention: Accounting Dept/Royalties
3101 Hillsborough St
Raleigh, NC 27607-5436
A couple of years ago, Apress of Berkeley (CA) published a book of mine. I am an Australian citizen living in Australia, and the agreement between the USA and Australia sets to 10% the maximum amount to be withheld by a US publisher on behalf of the IRS. I read somewhere in lulu.com that only the royalties of copies sold within the US are subject to withholding, but Apress withheld 30% on all my sales. I went through the process of getting an ITIN from the US Treasury, which cost me more than AU$150 (~US$135). When Apress received my W-8BEN with the ITIN, they started withholding 5% instead of 10% as set in the agreement. This tells you that things in this field tend to depart from what you read in the "official" papers. In any case, I also decided to get back from the US Treasury the excess 20% that Apress had paid to them before receiving my W-8BEN. To do this, I had to file a tax return in the US. It was a pain, but I did get a refund from the IRS. Note that the refund was sent as a cheque and it cost me another $10 to cash it in. All in all, it still paid for me, but I am not sure that it would pay for most "non-US-resident aliens" who publish with Lulu using a Lulu ISBN. I believe that if you use your own non-american ISBN, Lulu does NOT withhold US taxes. Anyhow, if you want to know exactly how I did it, you can have a look at my website, where I explain all the details: zambon.com.au/writing/ustaxes/
Good luck, Giulio.