You can send the sum as a bank transfer to

Radar Sofia NPO
Allianz Bank Bulgaria AD
Sofia, 1000

IBAN:    BG59BUIN95611000603044
BIC:       BUINBGSF

or, alternatively, use the option operated by PayPal here:

If you press the button, it will let you choose whether to pay with a PayPal account or by a credit/debit card.

Please, remember to write the name of the applicant with the payment as well as your first choice for a period of stay.