Once matched, vPost will ship the package back to SG no matter you pay or not.

Don't worry about this. All you have to do is to make sure the "Processed Date" of the last package does not exceed 2 weeks from the first package. After all packages arrive at vPost, make payment all at once. This is called consolidation.

You will notice that when you make payment, those earlier packages that have been shipped back will not allow you to choose "buy insurance or not" because they have been shipped back already before you pay.