Unfortunately, once the application has been mailed, you can't add anything to it unless Sheffield contact you themselves to ask for additional documents.
You mentioned you sent payslips, but what about all the other required financial documents?
There are 3 main requirements to meet for the application and each one has specific documents you need to send. The relationship evidence is just one of these requirements:
- financial
- accommodation
- relationship
Financial
You MUST send (for Category A):
- 6 months of original payslips showing no less than £1,550 per month before tax
- 6 months of original bank statements showing the deposit of every single payslip
- a letter from his employer confirming length of employment, current salary, length of time earning current salary and type of employment
Optional but recommended as they may ask for it:
- latest P60
- original job contract
Accommodation
- evidence of a residence to live in (either a rental agreement or homeownership documents)
- letter of permission from the homeowner or landlord to live there
Relationship
- letter from sponsor confirming his support of the application
- original marriage certificate
- 1 or 2 photos of you together
- boarding passes from trips to see each other
- letters/cards sent to each other
- screenshots of email inboxes/Sykpe call logs/message logs etc. showing regular contact (1-2 sheets of paper max. per source of evidence)
Did you also print and complete the second half of the application form, VAF4a Appendix 2, which covers each of these requirements in turn? Your application is not complete, and cannot be processed without this form.