The simple formula for calculating EU import VAT due is this:
(Goods value + duty + shipping) * VAT rate of importers member country
If the first part is <= the minimum value it is treated as zero as it is not worth the costs for such little amounts of VAT.
In practise there are adjustments to that formula as certain costs are exempt (e.g. loss insurance), the goods themselves may be exempt or attract different rates (e.g. books, children's clothing, etc), or the import might be subject to reduced rates or total deferment (like if it will be immediately exported back out from the EU).
Note that is a harmonised EU rule so will apply to Finland as well - only the VAT rate is different between members. (It appears the calculator you have used on Finland's customs site is incorrect.)
Running the following through the EU import calculator using Finland as the destination (24% VAT) gives these answers:
1) Goods nill value + shipping cost of €25 = import VAT of €6 due on €25 taxable transaction.
2) Goods €20 value + shipping cost of €1 = no import VAT due on €21 taxable transaction.
Mouser, although often shipping directly from outside the EU, are registered in the EU for VAT and handle the import part themselves and supply you from their EU business (if you look at the shipping declaration you will see they use their EORI as the importer). They can then supply anywhere within the EU customs union without further tariffs, and VAT is collected on the final sale in the usual way. Its the same as buying from Amazon's US site and having it delivered to you in Finland - the shipping declaration will list Amazon EU Sarl as the importer and you as the destination address.
DHL, like most (all?) express carriers complete customs clearance on route the moment the package has left the destination country. The clearance process is often indicated on their tracking history list as being held in an actual warehouse in the destination country and then released, but in reality it can still be in the air somewhere near the equator. Sometimes you will then find a later tracking update showing the package is still in another country. If the tracking history shows a charge was raised then you will receive a letter in the coming week (DHL will send an SMS text message too with a payment code if they have your number).
In the UK about a decade ago when online shopping was first getting popular individuals suddenly found themselves importing from outside the EU customs union there was a lot of anger about carriers charging for the use of their deferment account and customs clearance/broker services. People were successfully arguing that the carriers must not hold packages hostage until payment was received under our UK laws, but had to release the packages and invoice separately. (The duty + VAT was always payable upfront without a valid deferment, and still is.) As the importer did not enter into any first hand contract with the carrier to provide their broker services the carriers were largely unable to enforce fee collection under contract law. Our laws were then updated to allow the carriers to legally hold packages until their reasonable fees where paid (some still choose to, most will invoice later if you ask them to). The fact that the UK was able to legislate domestically for that suggests that each member state can set their own rules there so the Finnish rules on paying the broker fees may well be different.