Congrats! I love getting cute new fuzzbutts in the mail
How big are the slings? I have an LP sling that I got at .25" and it grows fast
! When mine was .25" I kept it in a baby food jar (with small holes punched in the lid) with a decent amount of substrate that I made sure was moist but not wet. You can also use plastic vials, small deli cups, etc. that you can get for free very easily! I would moisten the substrate about once every 2 weeks or until it was bone dry again. I fed the LP pinhead crickets until it was about .75", at which point it got smalls. LPs are terrestrial so as it grows, floor space/width is more important than the height of the container it's in. My LP has been very hardy and eats like a champ!
I've also raised Avic avic slings from .25", and from what I've read, care for this genus is fairly uniform. Avic slings tend to be not as hardy as some other species. Out of the 5 I've had, I lost one randomly (we call it SADs...Sudden Avicularia Death Syndrome, where young Avic slings apparently up and die for no reason), and I lost another one because I suspect I kept the humidity too high, and mold started growing. Since then I've kept my Avic slings the same as my other slings (keep substrate moist but not wet, let substrate dry out before re-moistening). Good ventilation is also very important for your A. metallica, so make sure you have plenty of air holes in whatever container you use (putting holes in the top and the sides of my containers has helped me & my pink toes a lot!). Since the Avicularias are arboreal, as your sling grows make sure your container has more height than width. Once mine were over 1", I moved them into larger vertically-oriented containers that had some fake leaves & cork bark that they climb & build tube webs on. Good luck and enjoy watching your new babies grow!