Nothing to do with Brexit and as long as the goods being shipped from the Uk were made in the Uk this tariff should not be applied, The Uk has an agreement that bypasses this.
My son sells goods that he makes (from my original patterns) into the EU and he has applied for exemption. Its just all the lazy business people importing from china and selling on where the issue lies.

I really wish people would stop blaming everything on Brexit, yes it was a pain and I wont buy Heinz salad cream cause its so bloody expensive now (silly really as the stuff is made in Elst and I know this for a fact as I moved the machines from the Uk there years ago and set up the factory). But its not the cause of every single little issue.

The new €3 flat customs duty is a global rule applied by the European Union to all third countries (non-EU nations) across the world.
The primary target of this reform is the massive influx of ultra-cheap e-commerce shipments originating from Asian manufacturing hubs—specifically targeting giant Chinese marketplaces like Temu, Shein, and AliExpress

While it was not created because of Brexit, Great Britain is affected by it because it is no longer an EU member:

• The Third-Country Rule: Because the UK left the EU Single Market and Customs Union, it is legally treated as a "third country". Therefore, any low-value online orders shipped directly from Great Britain to consumers in the EU (such as Ireland or Spain) are subject to this €3 fee.
• The Rules of Origin Exception: The EU-UK Trade and Cooperation Agreement (the Brexit trade deal) allows for zero tariffs, but only on goods that genuinely originate (are manufactured) in the UK. If a UK business operates as a fulfillment center shipping cheap items manufactured in Asia to EU consumers, those parcels will face the €3 fee.

In summary, this is a broad EU-wide e-commerce customs reform targeting global retail models, but Brexit means British exports to the EU happen to get caught in the same crossfire