My company has been using eFulfillment Service for about 5 years now, with volume of about 3,500 parcels per year. We've had our ups and downs with EFS over the years. Early on EFS had very slow ship times, with regular ground shipments usually taking 2 to 3 days to ship after they received an order. Fortunately in 2016 they greatly reduced that to same day shipping on ground orders that came in by early morning.
Important for anyone linking their shopping cart to EFS (almost everyone), EFS does not offer two-way XML data exchange with carts, so your cart can send them orders, but they cannot send stock data back to your cart to keep stock levels current in your shopping cart. However, you can hire a coder to write a custom XML app to do so if you have the money.
One area that EFS does well at is the variety of carriers and shipping types offered, particularly with lower-priced overseas shipping options and 1st class mail for small inexpensive light items. The latter has allowed us to beat the competition on shipping costs for small low-priced items.
Historically, we had very little loss from stock missing or theft, but that increased significantly in 2016, with losses apparently from theft or loss of stock, not accounting errors. EFS does pack shipments well so we had very few losses from damaged shipments. Stock accounting errors have become more frequent, with significant losses that we had to haggle over being reimbursed for, and having to provide documentation to prove quantities shipped to them. While ship times improved greatly and satisfactorily in 2016, stock check-in times became ridiculously slow, with a week to 10 days to check in our stock being the norm since August of 2016. We were initially told these very long check-in times were due to the pre-holiday season approaching, but those excruciatingly slow check-in times have continued into March 2017. EFS is apparently undergoing some sort of staffing crisis, because they have given up on even responding to our requests to get our shipments to them checked into stock. I'm looking for a new fulfillment service since we can't afford to plan on 10-day dock to stock times. I suspect EFS is too small to compete effectively with the big boys. I also suspect many of the glowing 5-star reviews elsewhere were planted by EFS, and small businesses who don't have much experience with fulfillment services.
Extremely slow stock check-in times, 7 to 10 days being the norm as of March 2017. Lack of 2-way XML shopping cart integration, so stock inventory data cannot be sent to shopping cart.
I would check with your shopping cart support to see what fulfillment services it integrates with natively, and make sure the fulfillment service offers two-way XML data exchange, so as you grow your cart and web site can show consumers stock levels before they purchase, preventing back orders and greatly reducing staff time spent manually adjusting stock levels in your cart. Fulfillment services always quote glowing dock to stock and ship times, confirm with existing customers what the reality is.