Can I let customers buy more than one product or choose more than one subscription plan?

Yes, toggle either question's "Can buy more than one" or "Can choose more than one" option in the question's configuration.

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Can I let submitters choose multiple answers for a multiple-choice or dropdown question?
Yes, Multiple Choice and Dropown fields both support multiple answers. You can turn this on by toggling "Can choose more than one answer" in the question's configuration.
How do I add and configure products on my form?
Our Product Field is used to capture the purchase of items and can be configured so that different products can be purchased, and in different quantities.
How do I add or remove subscription plans?
In a Subscription question's configuration, click on a plan to move it from "Available Plans" to "Selected Plans" or vice-versa.