I swearrr whenever I try to go combine some items they're always invalid.. are there a list of items which combine? or is it random?
There is a list of the items for you to make.
Check the SubetaHQ for the list of items.
Blob Kitty + Death Shard = Death Kitty
I did 10 lots of that, and got all three acheivements
I did the Not A Noob Picket Sign and it cost me under 10k to create each one.
+
They may have inflated since then though.

Making a death kitty is too expensive :o
I made a couple of Antihero Sword and you need to do another combo to make that one
Most of the items used to create things have inflated since the achievements came out. Try restocking items needed at the Stable Smithy to get them cheaper. I made a bunch of swords that way (:
btw, it's n00b with zeros and not o's ^_^
I made Weenies! : D They weren't too much to make.
in shop for ~1650 and in shop for... something like ~450. Nice, nice. User shops are more, of course, but if you wait, they'll rs.
great tip, thank you for that! it was relatively inexpensive
Thanks lapis... And thanks to all of you for the great suggestions for the uninitiated in the Genetech lab.
I used and
created

Thanks froggy. i used that combination too. i got a few of them. I wanted the cutesy beanie so i got that combination too.. thanks everyone for the help getting all three achievements...
thanks guys. I used the N00b Picket Sign and Sun Marker to get the three achievements
I've been combining plain pumpkins left over from last Morostide and the flavored syrups that are sold at SubetaBucks :) Seems to work out well too.