Welcome to Spain - which is not exactly the free world!
If it's only samples and not for re-sale then they should arrive ok and no problems generally re duty/vat.
No problem receiving goods generally from Hong Kong/China/US etc
I think the import agent is only required if you wish to import for re-sale etc and then you would need to comply with all type of rules and regulations which means you will never get your business off the ground!!!!! I know some people who ran into problems after they had imported goods and then found they could not legally sell them etc so be cafreful and do your homework.
I was in Calahonda last week watching the Policia Local harrass all the shop/bar/restaurant owners for incorrect use of terrace space. Fine them and put them out of business that type of thing and then totally ignore all the lookie lookie men who take over all the footpaths seeling their counterfeit items. Sorry I rant!