There is nothing I dislike about their service as everything has been positive so far! We are using them for our order fulfilments but we do also use Shipwire, which seems quite similar in every way except when it comes time to pay out each invoice they want you wait 30 days before paying your entire balance owed (even if you paid only 1 month's worth).
It just makes things easier that there was no such waiting period since day one with ZF. Please be sure yo utilize this software early because once all orders hit ship date, those invoices will get very high VERY quickly without any discounting even at full rate price; especially given how low rates were pre-COVID crisis/lockdown times compared to other carriers like FedEx & USPS - not being able too take advantage now would seem foolish indeed!!