The OP of the question eCommerce opensource plattforms seems to look for software "in order to integrate with". So OP is not looking for software that they could use themselves.
Should this and similar questions be on-topic?
I think some arguments from Is looking for software as “data” within our scope? and Is asking for sample, illustrative code on-topic? can apply in such a case, too (but the mentioned discussions are not exactly about this case).
The question Is asking for lists of software welcome? may also be relevant (while OP is not explicitly asking for a list, I think a list would generally be more useful to OP than a single, even if very good, recommendation).
Update: Just to be clear, this discussion is not about asking for libraries (which is on-topic): in the case of libraries, the OP is using the recommended software. In the cases this discussion is about, OP is developing for the recommended software (e.g., creating a module for various ecommerce CMS to offer integration with their payment provider).