If you long press lightning bolt you can set a custom zap/sats amount...

FWIW I finally figured out how to zap with wallet of satoshi and have been spreading the love. I use Amethyst on an old android phone.

These are the steps I took:

1. Download wallet of satoshi

2. Open and click "recieve"

3. Tap the "@" and then tap the QR code to copy the Lightning URL to copy the LNURL address

4. Open Amethyst and open your profile

5. Click the edit icon and paste your clipboard content to the LN address box, then close.

The zaps worked after that!

Same here!