First of all, you can't transfer $20 with the bitcoin protocol.  You can't transfer U.S. dollars at all.  You can ONLY transfer bitcoins.

I assume you are asking about transferring $20 worth of bitcoins?  But, since the exchange rate of bitcoins is constantly changing depending on current demand in the market, the actual amount of bitcoins will be different now than earlier or later.

For the sake of discussion, lets go with an exchange rate of $14500 per bitcoin.

So, what I think you are actually asking is:

"If I want to send 0.00137931 BTC (1.37931 millibitcoins) what would your suggested transaction fee be to make sure the transaction confirms?"

The answer to that question is also constantly changing, and it depends significantly on how you received those 0.00137931 bitcoins.

That being said...

Based on this chart:
https://jochen-hoenicke.de/queue/#2h

It looks (given the network conditions at the time that I am writing this post) like you can expect to get your transaction confirmed if you pay a fee of at least 0.00000140 BTC (0.00140 millibitcoins) per byte.

Lets assume that you previously received a payment of exactly 0.00164811 BTC to a traditional P2PKH address (address that starts with a 1) which you have not yet spent.  In that case, you could expect to be able to create a transaction sending 0.00137931 BTC which requires no more than 192 bytes and therefore a transaction fee of only 0.00026880 BTC (approximately $3.90 worth of bitcoins at the exchange rate of $14500)

If you received a slightly smaller payment to a SegWit address, then you could pay an even smaller fee.

If you are willing to wait a long time for confirmation (a few weeks), you might be able to pay a fee as low as 0.00000100 BTC per byte (only 0.00019200 BTC in the above example).