I want to connect Arduino Due with this module via tx and rx: http://aliexpress.com/item/Official-Smarian-Mini-USB-16-road-servo-controller-servo-control-board-USC-16-servo-drive/32796976115.html
So, do I need ttl level converter 3.3v <-> 5v ?
I'm just wondering, because it seems I dont need a level converter for sd card module
It depends on the whether that board can reliably can accept a 3.3V signal at its serial input. It does say "TTL", which usually means under 0.8V is low and over 2.0V is high. But then there's always some risk in buying cheap stuff from Aliexpress.
Did you try asking the seller on Aliexpress whether their product can work with the 3.3V serial signal from Arduino Due? While you're at it, you might also ask them for a link to the info about what sort of data is expects you to send to control those motors. Without that info, I do not see how anyone could interface this with any microcontroller. But maybe it's a clone or knock-off of some other well-known (but not known to me) product and it listens for the same inputs?
If you get a reply from the seller, I hope you'll share it here. Always amusing to hear what those Aliexpress merchants offer as "support" for thier product!