amazon is charging you for pressing your joints but since they stock your real breadwinner the comic stay with them but there is no reason you shouldnt have your own website and do a variation of your owen exclusive tees and get your own packaging for cheap too.
600 is a great price, but you are in the USA if you order 250 menswear and 250 womans wear thats a great price easy to stock thats like 4 boxes. Im in the UK so i got a post office account and most my tee's are abroad so i was always wanting to move into sweat tops thats the real cash. its cold here people wear sweaters.
^ i got boxes of this exact replica grey sweat with my logo embroided on the top left breast. thats me branching out but its just sitting there till i have enough to push it. as for you i would do the same you are an an illustrator you can design dope tee's.
i would even do practice samples and go to your local market stalls and malls and tell them you will take commision if they stock your product. use instagram get them girls to wear your shyt. many more things i do to stay ahead.
but i lost some accounts start of the year (i think thats because i only do tee's and scarves on my print) i do customizinng gear (which you should do) on the side, but for your label i would jump all in it. get the best quality fabrics and tools and paperwork.
go to your local town hall with your shyt and a business plan and get a grant. Im not dropping anymore intelectual property because im trying to eat i even have to punch it the clock to cover rent
while im saving for a bigger push into the market.
people are spending online because all purchases are now protected, so set up shop bro.
you actually lucky you dont have to out source any work and your resources are cheap once you buy the bulk tee's and a good heat/silk presser then its just ink and your spare time to carve up some of them dope illustrations.
good luck.
and since you are only doing tee's for merchandise just buy an iron and use transfer paper, thats the cheapest and best alternative to copping a presser.