In fundraisingManager 2 you can have a donation form with a single option. This can be used for memberships or other fixed amount solicitations.
You can still offer campaign options, tributes and allow the constitutent to cover the fees.
Set the Donation Amount Mode to "Single Amount"
Ensure that there is only one donation amount (one-time or recurring) that have Default checked.
On the selected Donation Amount the text in the Description field will be displayed.