If I do not pay fees at the time of filing of my application, can I pay the fees through EFS-Web later?

Yes, if you are a registered e-filer, you may pay your fees later as a follow-on submission. However, if you are filing a national stage application under 35 U.S.C. 371, the basic national fee must be paid no later than 30 months from the earliest claimed priority date; otherwise, the application is abandoned.

USPTO FAQ - Questions and Answers (23NOV2007)