Currently I'm with a 3PL and I think the cost is hard to justify. Forget the storage fee, but it's about $3 for pick/pack/packaging + another $4 for shipping. So my total fulfillment cost is about $7-$8 although it's less than 1oz.
With Amazon, I'm not sure if I understand their policy, but they don't charge a shipping fee. Fulfillment fees for Multi-Channel Fulfillment orders – Amazon Seller Central
So for <2oz, it's $3.99. I assume this include the shipping cost + the packaging/pick/pack. Almost half the amount.
At this stage in my business, the branding isn't as important but rather being able to breakeven/profitable.