There is a normal process in place for importers/brokers to request refunds if a specific tariff was overpaid or a tariff was ruled to be illegal.
But if you imported through DHL and you were not the broker, that is more complicated, you might need to ask DHL for it, and they might not want to do it for you (as they don't have a standard process in place).
I spent a bit of time attempting to find a broker [1] to handle this for our project (since we had a large amount of eligible refunds due to importing then sending out of country after QA) but in the long run gave up...which is what they hope for.
Keeping an eye on all this to see how it plays out.
[1] Not only did I look for a broker but I debated becoming one myself due to this.
i.e. Where you upload your paperwork, fill in and certify the forms online, make a payment, and the broker just feeds all that through. You do the work, they're just your gateway to the system.
I've used courier's internal brokers (like DHL/UPS offer, at their ripoff rate), professional private brokers, etc. and seen all of them make stupid mistakes costing me money/time (eg. including the shipping cost in value for duties, transposing the wrong currency at face value, etc). I could do a better job myself, and frankly with a decent portal it would take me less time. Heck I bet I could build a fairly automated system that is more efficient (higher-margin) and accurate.
Here in Canada there's new legislation that even if you use a third party broker, you still need to post a security or bond with CBSA (see CARM) maintained on an annual basis. It boggles my mind they made the infrastructure to deal with money from all the individual buyers, but not a self-service portal to deal with the forms. Self-clearing here still entails a physical visit to a CBSA office.