Hello Issac,
Method does not sync with custom fields created in QBO. It is a limitation of the IPP (ie. Quickbooks Online) and not Method. We are hoping for an update soon from Intuit. You can read our Help Center guide on the topic here.
For your second question on the memo field. In Method, the memo field is located under the Items Grid, just under Customer Message. This field links to the Statement Memo field on Invoices in QBO.
If you have any more questions, let me know.
Memo field in Quickbooks Online:
In Method: