Help sending bubble mailer from USPS to Europe

I want to send something to the UK and also possibly Sweden. Through research, I *think* a small bubble pack using regular mail might work. These are the conditions:

1. Want to use the cheapest method
2. Sending one or 2 small electronic boards
3. Total weight including envelope would be 3-4 ounces (85-113 grams)
4. Should fit into a small bubble pack or poly mailer with some foam inside
5. Value of the parts in the pack would be $20-$30
6. Will only do this on rare occasions (perhaps once or twice a year, if that)
7. Will fit in a 5x7" poly or bubble pack mailer

Some questions:

A. It makes no sense to send this if it costs more than $3-$4, might as well just send money and have them find these parts. Can I still meet that price?
B. Do I need to fill out customs forms even if I label this a gift? (Especially for the UK shipment since Brexit ruined the UK for business).
C. I'm worried about the boards being seen as some product I am selling rather than the gift (parts to experiment with) that they are.
D. Do I need to worry about bulkiness? I read somewhere .75" thick or less, which it would be in the middle of the pack.