- This may not be exactly what you're looking for, but this is my recommendation for investing small amounts of $ - either 1) use an app like you mentioned to track spare change or 2) choose a small amount of $ to invest per week. Then, set up a recurring buy for a cryptocurrency of your choice on Coinbase for that amount and check back in a year or two :)
Coinbase is there to get you started; it's easy to use and understand what you're doing. The transactions feel safe enough, and it protects you from buying too much at once. It genuinely seems like they take care of their users, especially new ones.
I have been waiting for a week now for my funds to become available on coinbase and they still are not. Plus, I have had zero feedback from the support team for a week now. Do not put your money here. Seriously.
- Robinhood is available for both iOS and Android. My friends and I have used Robinhood to invest small dollar amounts and test the waters for new investment ideas. Since Robinhood doesn't have fees, you can still trade spare change without worrying about losing money on every transaction.