I've been looking at many Instagram schedulers. Unfortunately, only a few actually post the pictures. And if they do, they cost a fortune - monthly. BUFFERGRAM is different: there is no compulsory monthly plan, you have the option of buying credits. For every credit, you can schedule and post one photo. That makes it attractive if you use scheduling just from time to time.
Disclaimer: I'm not associated with BUFFERGRAM, just a customer and fan.
