I just upgraded from j2store to j2commerce. I have 4.0.19 installed. I use the stripe plugin and I have updated that plugin as well.  Right now the plugin has api data that is the test data for Stripe.  However, when I proceed to payment, I get this error: 

The amount must be greater than or equal to the minimum charge amount allowed for your account and the currency set (https://docs.stripe.com/currencies#minimum-and-maximum-charge-amounts). If you want to save a Payment Method for future use without an immediate payment, use a Setup Intent instead: https://docs.stripe.com/payments/setup-intents   If you want to test one item, you can try this one, for examploe: https://eukmamie.org/new/new/es/catalogo/otros/libros/14517-valeria-el-cielo-se-lleva-dentro#adquirir
The cart is then found here: https://eukmamie.org/new/new/es/cart

Thanks!

I have continued testing - also with the paypal plugin - and it gives an error saying that the amount is missing. So, it seems that for some reason J2commerce is not sending the amount to the payment plugins...