How to Successfully Scale Up Tee Shirt Sales with FB?

I'm a late entrant to the tee shirt bandwagon on both Sunfrog and Teespring. Conversions are 1-3% on Sunfrog. 3.8% on my first "successful" (8 shirts) Teespring launch.

With Teespring, I've had one successful launch, but lost $1 with all things considered.

With Sunfrog, I cannot seem to scale up successful campaigns. I'll start with a $20/day buy and make $40 for a $20 profit. I'll scale up to $40/day and make $45 or $39 or $28. It doesn't make sense. The more I spend, with the exact same market, on the same days, the less profitable it is.

I've targeted within my niche, but broader targeting seems to work better. (basic keywords, age 20-65, male & female) I've targeted based on the ages and gender that engages, but conversions drop. What the heck?

Boosted posts seem to work much better than ad buys. They also build my likes, although I'm pretty convinced likes are almost meaningless anymore.

I am positive FB artificially limits organic reach. I know since my 4 million/week dropped over 95% the day after my first ad run finished and never recovered. Now, my big weeks are more like 150,000-200,000.

I'm getting the feeling FB is limiting ad reach to maximize how much you spend. Like finding the pain threshold. I tried boosting posts for smaller pages and the return is laughably low.

I'd have to spend $3,500+/week to see $1,000+/week profit - and that's if I have all winning designs. I'm not willing to risk it until I get a better understanding as to why spending more on FB seems to yield less...

Any thoughts? Thanks!