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.

I believe the email you mentioned as " 要求付钱的邮件" is titled "vPOSTUSA Shipment Alert".

You will receive shipement alert from vPost for EVERY package only after it is processed. Sometimes you may find the package is received by vPost but it uaually takes 1-2 more days for them to match and process. After processing, they will send you shipment alert letting you know the weight, etc but just ignore it if you have other packages to consolidate.