is that correct?

If they filled out the application and paid the fees by check or cash (cash or check)within the first week of the enrollment period, fees for members will be 300 dollars and non-members will be 350 dollars. If they finished the process after the first week of the enrollment period, fees for members will be 350 dollars and non-members will be 400 dollars.

If they submit their application forms and pay their fees either by cheque or by cash in the first week of enrollment, members will only pay $300 and non-members will pay $350. If they submit after the first week of enrollment, they have to pay $350 for members or $400 for non-members.

This is the addition from the daughter of my host family in Australia :
If they completed the application forms, and paid the fees by cheque or with cash within the first week of enrollment, members will only have to pay $300 and non-members will have to pay $350. If they do not complete the application form and pay the fees within the first week, members will have to pay $350 and non-members $400.

There are many mistakes.....

Try to correct the above as:

If you return the finished application forms and pay the fees (either by cheque or cash) in the first week of the enrolment period, the application fees will only be $300 (for members) and $350 (for non-members) ! After the first week the fees will be $350 (for members) and $400 (for non-members).

Not really correct,it should be
If they fill up those application forms and pay fees by check or cash in the first week for the enrollment,one membership only pays 300 dollars, non- membership only pays 350 dollars,if they finish it without the first week for the enrollment,one membership only pays 350 dollars and non-membership pays 400 dollars